compressed texture format against WebGL 2 are not supported on MacOS and Desktop GL

ETC2 and EAC Compressed texture formats are core feature in ES 3.0. But they are not core feature in OGL 4.2 or below. Only OGL 4.3 or above can support these compressed texture formats. So tests for these formats failed if the underlying OGL is lower than 4.3.

For instance, On my MacOS laptop, which is 10.11 on the basis of OGL 4.1, many test cases in conformance2/textures/misc/compressed-tex-image.html and conformance2/textures/misc/tex-storage-compressed-formats.html failed in Chromium (and Firefox). These two test files test all ETC2 and EAC compressed texture formats for compressedTex{Sub}Image2D and texStorage{2|3}D.

Any thoughts about this? Do we have some workarounds for OGL? If not, seems that we should skip the test cases and do not expose these compressed texture formats as WebGL 2 core feature, but expose them to developers as WebGL extensions.

Start with conformance2/textures/misc/compressed-tex-image.html and conformance2/textures/misc/tex-storage-compressed-formats.html, reproducing the failures on the reported MacOS OpenGL 4.1 environment. Compare the ETC2 and EAC cases with WebGL 2 and OpenGL support; completion depends on an agreed workaround or on defining which cases should be skipped or exposed as extensions.

Written by the indexing model from the issue text.