GDK-Win32: Add fallback mode to GLES

...if GLES (libANGLE) support was enabled in the build.  This way, we can
check whether the GL driver is capable enough to support the OpenGL
features that we use in GTK.  If the driver is not capable enough, and
GLES support is enabled, we can try to create the GdkGLContext again as a
GLES context.