When running on gtk we need X11 platform not mesa platform.
Create separate functions for mesa and x11 so we can keep
the egl #ifdef mess local to egl-helpers.c

Fixes: 0ea1523f
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170505104101.30589-4-kraxel@redhat.com