GDK-Win32: Backport MR !5702 into gtk-3-24

Hi,

This attempts to backport !5702 (merged) into gtk-3-24, since I think it would be the best to make things safer for people that are attempting to use gtk-3.x with GdkGLContext with items that also use WGL contexts outside of GTK and/or across different threads.

Like what is done in gtk-4.x, we have the needed wgl*() calls linked directly to the system's/ICD opengl32.dll instead of going through libepoxy, so that we avoid crashes that are caused when items outside of GTK/GDK make the WGL context current, as the function pointers acquired by libepoxy for GTK/GDK can well be rendered invalid.

With blessings, thank you!