GL_ARB_ES3_compatibility brings ETC2/EAC formats to desktop GL.

The meaning of the GL compressed format list is pretty vague - it's
supposed to return formats for "general-purpose usage".  (GL 4.2
deprecates the list because of this.)  Basically everyone interprets
this as "linear RGB/RGBA".

ETC2/EAC meets that criteria, so while we shouldn't be required to add
it to the list, there's also little harm in doing so, at least on
platforms with native support.  I doubt anyone is using this list for
much anyway, so even on platforms without native support, it's probably
not a big deal.

Makes the following GL45-CTS.gtf43 tests pass:

* GL3Tests.eac_compression_r11.gl_compressed_r11_eac
* GL3Tests.eac_compression_rg11.gl_compressed_rg11_eac
* GL3Tests.eac_compression_signed_r11.gl_compressed_signed_r11_eac
* GL3Tests.eac_compression_signed_rg11.gl_compressed_signed_rg11_eac
* GL3Tests.etc2_compression_rgb8.gl_compressed_rgb8_etc2
* GL3Tests.etc2_compression_rgb8_pt_alpha1.gl_compressed_rgb8_pt_alpha1_etc2
* GL3Tests.etc2_compression_rgba8.gl_compressed_rgba8_etc2

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>