On Windows, GLES is not that widely available unless one installs wrapper
libraries such as libANGLE, so GLES/EGL support on Windows is used more like
a fallback mode if Desktop OpenGL (WGL) support is inadequate on the system.

Hence, unless one forces WGL or EGL, we will first try to initialize WGL, and
then try to initialize GLES if enabled and if WGL initialization failed, and
then just return whatever the last result we can obtain from these
initialization attempts, since unlike X11 EGL contexts, we do not have
separate modes for WGL except for legacy and non-legacy contexts.

We were setting the WGL pixel format in GdkWin32Display too early, so the code
does not bail out correctly when we retry establishing the WGL context.

Fix this by pushing back setting the WGL pixel format only after it passes the
shader availability check.

Should fix issue #4257.