Added support for EXT_texture_sRGB_R8. The GL_SR8_EXT format
will be available if the VK ICD supports VK_FORMAT_R8_SRGB.
This does not add support for emulation of this format.

Bug: angleproject:4503
Bug: angleproject:3609
Test: dEQP-GLES3.functional.texture.*.srgb_r8_*
Change-Id: Ifaec92ac1a8561881f54ddb7fdf0975112711736
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2135853
Commit-Queue: Brandon Schade <b.schade@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Tim Van Patten <timvp@google.com>