GL_EXT_texture_compression_s3tc_srgb wasn't being exposed on macOS
because formatutilsgl.cpp was looking for GL_EXT_texture_sRGB, but
it was moved to core in 2.1 and the macOS driver doesn't report it
anymore.

Bug: angleproject:4997
Change-Id: Iae736fac6d315f567ff5d834a043134f91625e59
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2380516Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>