Add support for OpenGL ES to egl-helpers.  Wire up the new option for
egl-headless and gtk UIs.  egl-headless actually works fine.  gtk hits a
not-yet implemented code path in libEGL when trying to use gles mode:

  libEGL warning: FIXME: egl/x11 doesn't support front buffer rendering.

(This is mesa 17.2.3).

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Tested-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Message-id: 20180618112141.23398-1-kraxel@redhat.com