Implement EXT_texture_sRGB_decode. This builds on the existing
functionality from EXT_texture_sRGB_override, with 2 major edge
cases:
1. sRGB_decode allows the texture state to be overridden by
sampler state, which is implemented by forcing a a texture state
sync during updateActiveTextures if a texture is bound to the same
unit as a sampler with that state
2. texelFetch calls require us to reenable decoding, regardless
of decode state. We add a new compiler pass
(FlagSamplersWithTexelFetch) to mark samplers that are used with
texelFetch in order to support this.
This change also re-enables EXT_texture_sRGB_R8, which was disabled
due to a dEQP bug that this change will bypass.

Bug: angleproject:3609
Bug: angleproject:4503
Test: dEQP.GLES31/functional_srgb_texture_decode_skip_decode_*
Test: GLES31/functional_state_query_texture_*_srgb_decode_*
Test: GLES31/functional_state_query_sampler_*_srgb_decode_*
Test: GLES31/functional_debug_negative_coverage_*_srgb_decode_*
Test: GLES31/functional_android_extension_pack_extensions_ext_texture_srgb_decode
Test: angle_end2end_tests --gtest_filter=SRGBTextureTest.*Vulkan*
Change-Id: I4a67e487dc82e2f57c8c87d4bcd8ef442b6fe220
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2359481
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>