Those two callbacks only exist in wl_output version 2. VLC uses
version 1 only so far.

In practice, video output display plugins return an error when they do
not support arbitrary sizes. However nowadays, this control is requested
only when the video output or the video window report a change of size.
That cannot be allowed to fail: the display size is what it is
regardless of the result of the control request.

This enables OpenGL/OpenGL ES through Wayland.

At least the Wayland EGL backend needs to be notified of the size of
the window (which is independent of glViewport()).

This could conceivably also be implemented with a call to
glGetIntegerv(GL_VIEWPORT), but that would introduce a dependency on
the GL - which the EGL plugin has avoided so far.

If there is a window, it has to be used given the video output has non-zero
priority. Unfortunately, there are no good ways to run SDL within a window
(it can be done, but it is not thread-safe).

This is untested because vsxu is neither in contribs nor in my distro.

This control is only useful if the video output is called "opengl".