Fix GL_EXT_texture_compression_s3tc_srgb on macOS
Fix GL_EXT_texture_compression_s3tc_srgb on macOS

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/+/2380516
Reviewed-by: Jonah Ryan-Davis <682bdb9e7d8ba359ef169b8db374e7a99d394a46@google.com>
Reviewed-by: Kenneth Russell <1ebccd90c2bc76de65b6f830f2128a91ed69fdfb@chromium.org>
Commit-Queue: James Darpinian <eafe431c9f653c5da1c61a994876a786dff6f290@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle