gtk: add opengl support, using egl

gtk: add opengl support, using egl

This adds opengl rendering support to the gtk ui, using egl.
It's off by default for now, use 'qemu -display gtk,gl=on'
to play with this.

Note that gtk got native opengl support with release 3.16.
There most likely will be a separate implementation for 3.16+,
using the native gtk opengl support.  This patch covers older
versions (and for the time being 3.16 too, hopefully without
rendering quirks).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>