win32/gl: Ensure we use the 3.2 core profile

win32/gl: Ensure we use the 3.2 core profile

Emit an error if the profile is different.

This is a follow-up commit to commits cc45e82 (x11/gl: Ensure we use the
3.2 core profile) and 2d9081d (wayland/gl: Ensure we use the 3.2 core
profile), so that we do the same on GDK-Win32.  Update variable names and
comments so that the code is clearer to people, as we still need a
temporary legacy WGL context first before we can use
wglCreateContextAttribsARB() to create a WGL core (3.2+) context.

https://bugzilla.gnome.org/show_bug.cgi?id=741946