ui/gtk: Make sure the right EGL context is currently bound

ui/gtk: Make sure the right EGL context is currently bound

Observed a wrong context is bound when changing the scanout mode.
To prevent problem, it is needed to make sure to bind the right
context in gtk_egl_set_scanout_mode/gtk_gl_area_set_scanout_mode
as well as unbind one in the end of gd_egl_update/gd_gl_area_update.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230628191504.17185-1-dongwon.kim@intel.com>