Remove backend system and ctruLua support

Since I only use the LÖVE backend anyway, this simplifies the code.
Tidied some code.

input/axis.lua

@@ -0,0 +1,162 @@
+local input = require((...):gsub("axis$", "input"))
+local button_mt = require((...):gsub("axis$", "button"))
+
+--- AxisInput methods
+local axis_mt
+axis_mt = {
+	-- Axis inputs --
+	-- Axis input is a container for axes detector. An axis input will return the value of the axis detector the most far away from their center (0).
+	-- Axis input provide a threshold setting; every axis which has a distance to the center below the threshold (none by default) will be ignored.
+	-- @tparam AxisDetectors ... all the axis detectors or axis identifiers
+	-- @tretrun AxisInput the object
+	_new = function(...)
+		local r = setmetatable({
+			hijackStack = {}, -- hijackers stack, last element is the object currently hijacking this input
+			hijacking = nil, -- object currently hijacking this input
+			detectors = {}, -- detectors list
+			val = 0, -- current value between -1 and 1
+			dval = 0, -- change between -2 and 2
+			raw = 0, -- raw value between -max and +max
+			max = 1, -- maximum for raw values
+			threshold = 0, -- ie., the deadzone
+			triggeringThreshold = 0.5 -- digital button threshold
+		}, axis_mt)
+		table.insert(r.hijackStack, r)
+		r.hijacking = r
+		r:bind(...)
+		r.positive = input.button(function() return r:value() > r.triggeringThreshold end)
+		r.negative = input.button(function() return r:value() < -r.triggeringThreshold end)
+		return r
+	end,
+
+	--- Returns a new AxisInput with the same properties.
+	-- @treturn AxisInput the cloned object
+	clone = function(self)
+		return input.axis(unpack(self.detectors))
+		       :threshold(self.threshold)
+		       :triggeringThreshold(self.triggeringThreshold)
+	end,
+
+	--- Bind new AxisDetector(s) to this input.
+	-- @tparam AxisDetectors ... axis detectors or axis identifiers to add
+	-- @treturn AxisInput this AxisInput object
+	bind = function(self, ...)
+		for _,d in ipairs({...}) do
+			table.insert(self.detectors, input.axisDetector(d))
+		end
+		return self
+	end,
+	--- Unbind AxisDetector(s).
+	-- @tparam AxisDetectors ... axis detectors or axis identifiers to remove
+	-- @treturn AxisInput this AxisInput object
+	unbind = button_mt.unbind,
+	--- Unbind all AxisDetector(s).
+	-- @treturn AxisInput this AxisInput object
+	clear = button_mt.clear,
+
+	--- Hijacks the input.
+	-- This function returns a new input object which mirrors the current object, except it will hijack every new input.
+	-- This means any value change will only be visible to the new object; the axis will always appear to be at 0 for the initial object.
+	-- An input can be hijacked several times; the one which hijacked it last will be the active one.
+	-- @treturn AxisInput the new input object which is hijacking the input
+	hijack = function(self)
+		local hijacked
+		hijacked = setmetatable({
+			positive = input.button(function() return hijacked:value() > self.triggeringThreshold end),
+			negative = input.button(function() return hijacked:value() < -self.triggeringThreshold end)
+		}, { __index = self, __newindex = self })
+		table.insert(self.hijackStack, hijacked)
+		self.hijacking = hijacked
+		return hijacked
+	end,
+	--- Release the input that was hijacked by this object.
+	-- Input will be given back to the previous object.
+	-- @treturn AxisInput this AxisInput object
+	free = button_mt.free,
+
+	--- Sets the default detection threshold (deadzone).
+	-- 0 by default.
+	-- @tparam number new the new detection threshold
+	-- @treturn AxisInput this AxisInput object
+	threshold = function(self, new)
+		self.threshold = tonumber(new)
+		return self
+	end,
+
+	--- Returns the value of the input (between -1 and 1).
+	-- @tparam[opt=default threshold] number threshold value to use
+	-- @treturn number the input value
+	value = function(self, curThreshold)
+		if self.hijacking == self then
+			self:update()
+			local val = self.val
+			return math.abs(val) > math.abs(curThreshold or self.threshold) and val or 0
+		else
+			return 0
+		end
+	end,
+	--- Returns the change in value of the input since last update (between -2 and 2).
+	-- @treturn number the value delta
+	delta = function(self)
+		if self.hijacking == self then
+			self:update()
+			return self.dval
+		else
+			return 0
+		end
+	end,
+	--- Returns the raw value of the input (between -max and +max).
+	-- @tparam[opt=default threshold*max] number raw threshold value to use
+	-- @treturn number the input raw value
+	raw = function(self, rawThreshold)
+		if self.hijacking == self then
+			self:update()
+			local raw = self.raw
+			return math.abs(raw) > math.abs(rawThreshold or self.threshold*self.max) and raw or 0
+		else
+			return 0
+		end
+	end,
+	--- Return the raw max of the input.
+	-- @treturn number the input raw max
+	max = function(self)
+		self:update()
+		return self.max
+	end,
+
+	--- Sets the default triggering threshold, i.e. how the minimal axis value for which the associated buttons will be considered down.
+	-- 0.5 by default.
+	-- @tparam number new the new triggering threshold
+	-- @treturn AxisInput this AxisInput object
+	triggeringThreshold = function(self, new)
+		self.triggeringThreshold = tonumber(new)
+		return self
+	end,
+
+	--- The associated button pressed when the axis reaches a positive value.
+	positive = nil,
+	--- The associated button pressed when the axis reaches a negative value.
+	negative = nil,
+
+	--- Update axis state.
+	-- Automatically called, don't call unless you know what you're doing.
+	update = function(self)
+		if not input.updated[self] then
+			local val, raw, max = 0, 0, 1
+			for _, d in ipairs(self.detectors) do
+				local v, r, m = d() -- v[-1,1], r[-m,+m]
+				if math.abs(v) > math.abs(val) then
+					val, raw, max = v, r or v, m or 1
+				end
+			end
+			self.dval = val - self.val
+			self.val, self.raw, self.max = val, raw, max
+			input.updated[self] = true
+		end
+	end,
+
+	--- LÖVE note: other callbacks that are defined in backend/love.lua and need to be called in the associated LÖVE callbacks.
+}
+axis_mt.__index = axis_mt
+
+return axis_mt