GL_EXT_sRGB
http://www.opengl.org/registry/specs/EXT/sRGB.txt
GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING_EXT 0x8210
GL_SRGB_EXT 0x8C40
GL_SRGB_ALPHA_EXT 0x8C42
GL_SRGB8_ALPHA8_EXT 0x8C43