gdk: Add missing declaration in GdkGLPixelFormat
tests/glarea: Make the UI a bit more complex

Add sliders to control rotation angles on all three axis.
gdk/x11: Plug a leak in create_gl_context()

The valid GdkGLPixelFormat is owned by the GdkGLContext using it.
glarea: Clean up the ::realize implementation
gl: Drop the async viewport update

It is not the cause of the flickering, and after checking with the
current implementation of GL widgets like GtkClutterEmbed, it's not
necessary to update the viewport in response to a ConfigureNotify
for child windows.