Avoid to update ribbon widget too many times.

It fixes displaying screenshots in GP editor after switching between category tabs.
Merge branch 'master' of https://github.com/supertuxkart/stk-code
Simplify #include s
Simplify #include s
Simplify #includes
Simplify #includes
Simplify #includes
Fix #1554 (don't display server related screens on GUI for 0.8.2).
Fixed #1560.
Do not use matrix inversion for culling
Remove unused code
Merge pull request #1561 from marcoll/master

Some minor corrections
Use lesser rtt size for diffuse/specular
Fix another bug found while reviewing previous pull request
Fix linux compilation.
Merge branch 'master' of github.com:supertuxkart/stk-code
Fix bloom computation
Fix stk text billboard orientation
Disable old hack that causes more trouble than it's worth
Fix loading normal maps (new XML syntax)