The default ->upload_texture() works also for Windows since commit 27cf0fa,
as some of the problems described in 742953 also applied for GL core
contexts on Windows as well before 27cf0fa. Clean up the GDK-Win32 code a
little bit as a result.
This adds support for OpenGL to the GDK Windows backend using the WGL API
calls, which enables programs that uses the GTK+ GLArea widgets to work on
Windows as well.
This also adds a simple utility function to query for the version of OpenGL
that is supported by the Windows system, like the one provided by the X11
backend.
Many thanks to Alex (and Emmanuele, who started the OpenGL integration in
GTK+) who offered advice and help along the way, as well as the X11 and
Wayland backend for this work to refer to and to model upon.
https://bugzilla.gnome.org/show_bug.cgi?id=740795
