Compress/convert to srgb per material basis.

This allows to remove manual conversion in normalmap shader.
Fix skid mark color

We convert the vertex color value to srgb, optimally the vertex color
should already be corrected on cpu side but it would break fallback
colors...
misc little changes
Avoid the use of the new GP editor as a cheating aid
Merge pull request #1286 from marcoll/master

Avoid the use of the new GP editor as a cheating aid
Fix crash when loading instanced node without texture.
Use half res FBO for SSAO

SSAO is a medium frequency effect, using half res fbo is fine and
improve performance and look by doing an additionnal bilinear filtering.
Also tune up some values.
Bloom: Use power of two quads.
Bloom: Tweak the threshold.
Bloom: Weight the different sized contribution.
Apply MLAA after color_in.
Fix too dark screen with MLAA enabled.
Add an option for Tex compression

Also turn m_ssao into bool.
Add some more graphic config options.
Handle skybox texture compression.
Fix a memleak
Add a config option to disable dynamic lights
Remove the ability to disable pixel shaders

It doesn't work with GL 3.1 and mesa drivers (which requires a core
context)
Try to fix additive blend for object
Merge branch 'master' of https://github.com/supertuxkart/stk-code