After commit 447bc18c48 EGL on X11 broke.
But the handling of the GL context also was quite awkward because it was
unclear who was responsible for ensuring it got reset.
Change that by making gdk_gl_context_clear_current_if_surface() return
the context (with a reference because it might be the last reference) it
had unset, so that after changing EGL properties the code that caused
the clearing can re-make it current.
This moves the responsibility to the actual code that is dealing with
updating properties and frees the outer layers of code from that task.
And that means the X11 EGL code doesn't need to care and the code in the
Wayland backend that did care can be removed.
Related: !7662Fixes: #6964 on X11
This essentially reverts the changes from
c230546a2c but implies new semantics.
Namely, surface-attached contexts can now be bound to EGL_NO_SURFACE if
the windowing system isn't ready yet.
It is the task of the windowing system to make sure the context is
properly rebound when the contents become available.
We ensure this by checking in begin_frame() if we created the EGL window
and if we did, we make_current(). This works because creating the EGL
window creates the EGL surface and that does a clear_current(), so this
is always going to have the desired effect of re-making the current
context.
It is very convoluted though.
Fixes: #6964
Related: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11784
