Expose EXT_compressed_ETC1_RGB8_sub_texture on Metal and GL Updated the test to ensure that this extension is exposed on Apple GPUs when using Metal. Updated emulatedEtc1 limitation to hide this extension when the main ETC1 extension is hidden. Real WebGL apps cannot enable this extension because it is not exposed in WebIDL. Removed bogus entry from IsETC1Format and use the helper consistently in all validation functions. Simplified GetNativeCompressedFormat. Bug: angleproject:7471 Change-Id: I61321fadad7d962358d0fefecd08aaddaedd2ec2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3737762 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>