Group the WGL-specific code and GLES-specific code together, so that we
can reduce the number of #ifdef ... in the code, to make the code more
readable and easily maintained. This will pave the way to add a
fallback mode to use libANGLE (OpenGL/ES) in case the Desktop OpenGL
support is inadequte, if OpenGL/ES support is enabled in the build.
This is somewhat based on the updates that were done in GTK master, so
we are using one subclass for WGL-based GdkGLContexts, and another for
GLES-based GdkGLContexts.
Also remove the underscores in many of the functions in
gdkglcontext-win32.*.
Clean up the code a bit as a result.
9e949209a1