Allow PlatformEGLHeadless to load GLES

[PatRibas]

PlatformEGLHeadless only loads regular OpenGL, not GLES. This change allows PlatformEGLHeadless to also use GLES to help write tests against FL0.

BUGS = [462485141, 449755952]

[pixelflinger]

this looks pretty good, except for the requestES2Context change which I think is not necessary.

[pixelflinger]

Also please provide a better description for the change.

[PatRibas]

Also please provide a better description for the change.

Added a description to the merge comment in Github.

[bejado]

LGTM. Maybe also add a BUGS = [b/xyz] to the PR description to associate with a Buganizer bug.

[aWilson41]

Doesn't this mean EGL only ever works with GLES?

Tried to build headless but can't because it keeps giving me GLES.

Or is there some other pathway that supposed to be used to use EGL with regular GL? Or is EGL just not supported in filament for regular OpenGL?

New to filament just trying to figure out why it won't work.

[PatRibas]

Hmm, you should be able to still use regular GL with these changes, but reading this again it's possible something got mucked up. If you're unable to get it to work with BACKEND_OPENGL_VERSION_GL defined and need to make changes, feel free to open a PR and add anyone you see approving changes as a reviewer. Wonder if this is supposed to be something like || (defined(FILAMENT_SUPPORTS_EGL_ON_LINUX) && !defined(BACKEND_OPENGL_VERSION_GL).

[aWilson41]

I took a slightly deeper look and it looks like it goes a bit deeper than that since it branches off into the bluegl code and it doesn't seem to have any edge cases with EGL support or anything. Though the initialization is probably very similar I don't think that code is shared unfortunately and it seemed like much more work to remove GLX and X11 includes to get a EGL+GL without any GLX or X11. Prob just going to stick with Vulkan in this case.