Support non-SRGB capable GLX visuals

[quyse]

In commit 194aeb9 I had to remove SDL_GL_FRAMEBUFFER_SRGB_CAPABLE flag, as it was causing problems. It works without it, i.e. on all systems I have access to (Intel, AMD and NVidia graphics) the engine actually gets sRGB-capable framebuffer. Presumably there're sRGB-incapable systems however, so it would be good to properly abstract from this, by:

• checking if we actually got sRGB-capable framebuffer;
• adding gamma conversion shader function, doing nothing in case of hardware support for gamma conversion.