From the commit message, to fix running the gstreamer media backend on Windows when using the GL renderer, after MR !3027 (merged) was merged:
...for at least WGL, since we are not allowed to use wglMakeCurrent(), which is eventually called by gdk_gl_context_make_current() to share WGL contexts across different threads, which will cause a crash.
This means, we cannot enable WGL in the gstreamer media backend on Windows.
GstGL may not provide a way to connect the GdkGLContext to the underlying platform's native display system to the GstGLDisplay that it use (such as Windows), so check also whether we really have a GL context hooked up to the GtkGstPaintable that we use before attempting to realize the paintable. This allows the gstreamer media backend to continue to function without crashing when we could not connect the GdkGLContext to the GstGLDisplay directly.
With blessings, thank you!