Allow compositors to use desktop OpenGL

[emersion]

Some compositors might want to use GL features only supported on desktop GL.

I don't know if it's reasonable to use getProcAddress for all GL symbols and expect the GLES2 renderer to work with all supersets of GLES2. If not, we should probably leave desktop GL support up to compositors.

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wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1908

[emersion]

A few GLES features seem specific to GLES (ie. not available on desktop GL), and there are extensions that bring them to desktop GL:

• https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_ES2_compatibility.txt
• https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_ES3_compatibility.txt

It doesn't seem like we're using any of those atm.

[emersion]

This is actually something xserver/glamor does: https://gitlab.freedesktop.org/xorg/xserver/blob/master/glamor%2Fglamor_egl.c#L962