Show the DispmanX window only if there's no shared external GL context
set up. When a window is required by the context a transparent
DispmanX element is created and later on made visible by the ::show
method.
https://bugzilla.gnome.org/show_bug.cgi?id=746632
Depending on the platform, it was only ever implemented to 1) set a
default surface size, 2) resize based on the video frame or 3) nothing.
Instead, provide a set_preferred_size () that elements/applications
can use to request a certain size which may be ignored for
videooverlay/other cases.
I was lucky that:
e1 = vc_dispmanx_element_add ()
eglCreateWindowSurface (e1)
vc_dispmanx_element_remove (e1)
e2 = vc_dispmanx_element_add ()
resulted in having e2 equal to e1. And also having the egl surface
that does not allocate its buffers before the first draw.
