Add an EGL (OpenGL/ES 3) renderer for GTK+-4.x (using Google's ANGLE project) for Windows

Hi,

As part of the attempt to resolve issue #105, this is the code that adds an OpenGL/ES 3-based renderer for Windows using Google's ANGLE project, which translates OpenGL/ES calls into Direct3D 9/11 calls, for GTK+4.x. As in MR !191 (merged), this is activated via a use of an envvar for now.

As in MR !191 (merged) the part where we work out which GPUs we need to blacklist from using its OpenGL drivers (which may not be in a good shape) and fallback into using the ANGLE/OpenGL/ES 3 approach will most probably not be covered in this MR but will be done in another.

p.s. Am I correct about this...it seems that the Desktop GL and EGL renderers in GSK are not fully complete at this point, so there are some glitches and bugs that are present in both the Desktop GL and EGL renderers.

With blessings, thank you!