Mir Shells/Compositors Now Can Handle Desktop OpenGL

Up to now Mir servers (basically compositors / shells) have only supported OpenGL ES but now with the latest Mir work they are able to support full OpenGL.

This commit made today by Canonical's Daniel van Vugt allows for Mir's server platform to make use of desktop OpenGL rather than the OpenGL ES subset. Support for OpenGL ES is still retained and right now with this code it's still defaulting to GLES for Mesa drivers.'

This bug report dating back to early 2015 is about using full OpenGL for compositing. The motives for libmirserver/platform using full OpenGL are that Qt only supports full OpenGL on the desktop, not all proprietary drivers support OpenGL ES, some proprietary drivers perform better with their full OpenGL driver, and GLES doesn't work on some older hardware.

This bug has been addressed by today's work and the full OpenGL support will be available in Mir 0.24.