Add a fake skybox where no skybox is available

For spheremap shaders
Merge branch 'master' of https://github.com/supertuxkart/stk-code into devel
try to fix linux compilation
Revert to spheremapped reflection
Use correct mapping for spheremap
Fix texture sampled by spheremap
Spheremap: Flip y and add light.
Fix kart shadow vs kart animation (for the main player only)
Merge branch 'master' of https://github.com/supertuxkart/stk-code into devel
first try for a Random GP generator