mesa/main: Expose EXT_texture_compression_s3tc_srgb on GLES
This makes the following test:
bin/compressedteximage GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT
pass when emulating GL on GLES.
This makes the following test:
bin/compressedteximage GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT
pass when emulating GL on GLES.