Removed some unnecessary #includes in .hpp files to reduce compile time.
Merge branch 'master' of github.com:supertuxkart/stk-code
Minor cosmetic changes.
Fixed typo.
Merge branch 'master' of github.com:supertuxkart/stk-code
Clean some #define
Use non debug gl ctx in non debug build
Factorize matrix block uniform assignment
Add some documentation to add new material.
Try to fix OSX
Disable all extension on OSX

OSX does support GL 4.1 + a couple of 4.2 extensions. Since not all 4.2
extensions are present, disable extensions support instead of supporting
several combinaisons of extensions.
Inverted logic for compute shaders
Show "?" if a track has no screenshot (instead of the screenshot of
the previous track).
Fix particle for hd3000
Merge branch 'master' of github.com:supertuxkart/stk-code
Remove unneeded if (version/glcap) in shader ctor
Fix LOD
Merge branch 'master' of https://github.com/supertuxkart/stk-code
Move texture management function in a separate file.
Forgot to fix a particle shader for hd3000