gl: qglwtextureshare demo fixes and cleanup

gl: qglwtextureshare demo fixes and cleanup

1. glcontextid function is replaced by gstreamer gst_gl_context_new_wrapped .
2. call gst_init before gst_gl_display_new , seems gst_gl_display_new depends
   on gst_allocator_register , which only worked after gst_init called
3. flush gstreamer OpenGL context before using shared texture, fix
   flicker problem.

https://bugzilla.gnome.org/show_bug.cgi?id=735566