khr_texture_compression_astc] basic minor fixes

Just some minor stuff (one commit is even just optional) while triaging mesa#2280 (closed) for vc4.

For those interested on vc4, note that even with those changes, that test doesn't pass on vc4:

  • glCopyTexImage2D doesn't return INVALID_OPERATION with ASTC formats
  • glCompressedTexture3D fails even although GL_OES_texture_3D is exposed as supported.

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