[Mouse] Read mouse arguments out of Args table

Fix flamendless#20. Related to flamendless#80.

The function arg to customize screen to slab coordinates isn't passed
from Slab.Initialize -- it only passes the input table.

Usage with push.lua:

    function love.load(args)
        local function get_push_mouse(x,y)
            local new_x,new_y = push:toGame(x,y)
            if new_x and new_y then
                return new_x,new_y
            end
            return x,y
        end
        local game_width, game_height, window_width, window_height = 320, 240, 1280, 720
        push:setupScreen(game_width, game_height, window_width, window_height)
        args.TransformPointToSlab = get_push_mouse
        Slab.Initialize(args)
    end

    function love.draw()
        push:start()
            Slab.Draw()
        push:finish()
    end

Internal/Input/Mouse.lua

@@ -117,8 +117,8 @@ local function ProcessEvents()
 	Events = {}
 end
 
-function Mouse.Initialize(Args, TransformPointToSlab)
-	TransformPoint = TransformPointToSlab or TransformPoint
+function Mouse.Initialize(Args)
+	TransformPoint = Args.TransformPointToSlab or TransformPoint
 
 	MouseMovedFn = love.handlers['mousemoved']
 	MousePressedFn = love.handlers['mousepressed']