Cleanup skybox preparation code
Apply math07's patch for taking padding into account in height="fit", fixes #1762
MInor GUI XML cleanup
Add a check for GS support
Add codecheck before rendering GI/Shadow
Forgot UBO support check in shadow
Force disable shadows and GI when OpenGL is not recent enough. Fixes #1722
Merge branch 'master' of https://github.com/supertuxkart/stk-code
Fix gand prix
Fixed crash in kart selection screen
Use default track if remembered track doesn't exist.

It avoids issues in #1776 and probably other problems with addons tracks.