This thread dispatches navigation events. It is needed to avoid deadlocks
between window backend threads that emit navigation events (e.g. X11/GMainLoop
thread) and consumers of navigation events such as glimagesink, see
https://bugzilla.gnome.org/show_bug.cgi?id=733661
GstGlWindow_x11 thread is changed to invoke the navigation thread for navigation
dispatching, instead of emiting the event itself. Othe backends beside X11 do
not dispatch navigation events yet, but should use this thread when dispatching
these events in the future.
The navigation thread is currently part of GstGLWindow and not implemented in
separate subclasses / backends. This will be needed in the future.
gst_gl_window_x11_get_surface_dimensions is also changed to use a cached value
of the window's width, height. These values are now retrieved in the X11
thread, function gst_gl_window_x11_handle_event. This change is needed because
otherwise the XGetWindowAttributes gets called from the navigation thread,
leading to xlib aborting due to multithreaded access (if XInitThreads is not
called before, as is the case for gst-launch)
GstGLDisplayX11 holds the display connection and name. Each thread requires
it's own X11 Display connection (initialised from name) due to the fact that
we do not want to call XInitThreads(). Doing so would result in segfaults
when integrating with GUI toolkits Gtk, Qt, etc.
The Display connection is for OpenGL platforms where a constant display is
required in order to share contexts (egl). In the case of a wrapped context
(added later), we do not have GstGLWindow to retreive the display from so a
'master' connection is used instead.
