StdGL: Move gamma calculation to shaders if shaders are enabled

[Fulgen301]

GLSL shaders now do the gamma calculation. For this purpose, the gamma ramp is directly copied into a 256x3 texture.
The old way of setting the system's gamma ramp directly is still used for the non-shader code path for older systems.

TODO:

• 256x3 is a non power-of-two texture, which requires OpenGL 2.0 and won't be handled well by old GPUs. While those could just use the non-shader path, increasing the texture size to 256x4 at the expense of 256 bytes would solve that issue.
• Add error checking for texture creation - at the moment, the game will start with a black screen with no error logged even if it couldn't be created properly.
• DrawQuadDw looks weird with certain gamma values, e.g. the example given in the docs.
• Implement Graphics.DisableGamma properly - the shader currently still samples the gamma texture even if gamma is disabled. DisableGamma should disable any gamma-related code entirely.

[Somebodyisnobody]

[...] if it fails, the game will start with a black screen with nothing in the logs indicating an error.

Is this the desired or current behaviour?

[Fulgen301]

The current behavior - all other components (e.g. the shaders) log an error and exit the game if they cannot be created.