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".

The parameter is not "const vout_display_cfg_t *". This could not work.

Resizing is handled via the Refresh() function - only.