cairo: enable opengl for cairo-gl

cairo: enable opengl for cairo-gl

If OpenGL is enabled then enable cairo-gl.
However if GLESv2 is also present we must choose which implementation to
use.
Since normally OpenGL is more featured, has more dependencies and
wouldn't be present "by accident" it seems like the logical choice to
win over it.

This feature is useful for webkitgtk's accelerated 2d canvas.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>