Add SDL_GL_FRAMEBUFFER_SRGB_CAPABLE (SDL bug 1985)

Add the SDL_GL_FRAMEBUFFER_SRGB_CAPABLE GLattr now it's in SDL hg.

Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=1985
Commit: https://hg.libsdl.org/SDL/rev/4861edda71d1