EGL-backed GLX provider doesn't implement indirect contexts
This is somewhat tricky to add, because the object backing a pixmap under glamor is just an FBO and not an EGL surface, so you can't implement
glXMakeCurrent in terms of
eglMakeCurrent. You need instead to create your own non-window-system FBOs and use
glBindFramebuffer and friends.
I suspect this means we need to modify the generated dispatch code for some of the GL FBO API, which isn't quite trivial.
Also the above strategy assumes the DDX is using glamor. If we want to go all in on deleting the DRI driver loader we need to make this work even when the DDX is just fb, and the strategy for indirect contexts could very well be different in that case. (SwapBuffers could just be ReadPixels into the fb pixmap, etc.)