For completeness, added mappings of R8_SRGB and R8G8_SRGB
to OpenGL and Metal.

Bug: angleproject:4932
Change-Id: Ic8e44e3a94c114e985f6965fcd43fbcb8071432d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2880661
Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>