- glimagesink needs to be able to resize the viewport on aspect ratio
  changes resulting from either caps changes or 3d output mode changes.
- Performing a glViewport outside the GstGLWindow::resize callback
  will not have the winsys' stack of viewports required to correctly
  place the output frame.

Provide a function to request a resize on the next draw event from the
winsys.

Also track size changes inside the base GstGLWindow class rather
than in each subclass.

https://bugzilla.gnome.org/show_bug.cgi?id=755111