Add an EGL renderer for GDK-Win32 for GTK+-3.24 (was: merge request !126)

Hi,

This is a merge request to add an EGL renderer (via Google's ANGLE project) for the Windows GDK backend, for 3.24.x, as a part of the attempt to resolve issue #105. It is not totally ready, as the build system needs to be updated for this, and will also impose more dependency requirements for this as well. Please see the commit message on the current state of this work..

Similar work would also be done for the master/4.x branch once I am more confident of the work being done here, as the master branch is a bit too volatile for me to attempt to carry out the work there.

I think we could try to do the fallback mechanisms as things become clearer (such as LRN's observations that are pointed out in issue #105), which is likely to be done in another issue/MR.

With blessings, thank you, and cheers!