SSAO: Use a noise texture rather than a function
Use linear filtering for glow effect
texture2D is deprecated, use texture instead
Fix a crash with snow enabled tracks.
Undefine ARB_DEBUG_OUTPUT
Explicitly define fragment exported values.
Remove unused normalmap/splatting providers
STKMesh: Support grass shader
Use 8 samples instead of 16
Some welcomed factorization.
Added experimental change which allow to use alt-tab and other shortcuts on linux when game is run in fullscreen mode.

This method is used in many other projects and should work good. Currently modern games
use window manager fullscreen rather than grab whole mouse and keyboard.

Tested on Gnome, Unity and Openbox and it works well. Though it should be tested also on
other desktop environments.

Also performance impact should be tested.

I added simple workaround for Gnome, which sometimes creates fullscreen window smaller
than display.

If some problems will occur, feel free to revert it.
Merge branch 'master' of https://github.com/supertuxkart/stk-code
STKMesh:Another round of factorization.
Reworked the login and register dialog to be a screen - still
work in progress (and guest login does not work yet).