Changes to LOVE's scissors

Scissors now cut into the scissor above them, using relative x and y
positions. surface.SetScissorRect uses a breadcrumb system to create and
destroy scissors, so make sure if you use it you have as many blank
scissor calls as you do creations
Added MOUSE_ globals
Added input library for IsMouseDown macro
VGUI changes (read desc)

Moved vgui.mt into its own Panel file
Added vgui.RegisterInternal for built-in elements that may depend on
another that isn't registered yet
Elements and children elements now properly render with scissor bounds
Parentless elements now have z-indexes (PANEL:SetZIndex(int))
Mouse input will bring parentless elements to top
Added GAME global table, load lib.input and core.HardonCollider
Added AddDebugText and RemoveDebugText for overlay