Add extension EXT_sRGB_write_control (v7)
This series adds support for the GLES extension EXT_sRGB_write_control.
Earlier versions of the series have been discussed on the ML and can be found in patchwork: https://patchwork.freedesktop.org/series/51130/
v7: use _mesa_has_EXT_framebuffer_sRGB instead of testing the extension flag directly.
- renaming the new CAP
- reordering of the patches that no double checking of EXT_sRGB and EXT_framebuffer_sRGB is needed.
RFC: Complete rework of the earlier versions of the series, hence logging the history doesn't make much sense here.