mesa: GL_EXT_texture_sRGB_R{,G}8 enhancements

Also fix the extension date for sRGB_R8, initial revision of both was 2015. sRGB_RG8 is not registered for big-GL yet, see this Khronos issue for updates on that:

https://github.com/KhronosGroup/OpenGL-Registry/issues/450

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Mostly prompted by the above Khronos issue and because it seemed pretty trivial to add. Verified that this shows up now on softpipe and llvmpipe, and it passes u_format_test, but piglit doesn't have any coverage for this or the R8 sibling. Tagging the drivers that look like they could n atively support it.