sdlshare2: Fix race conditions

sdlshare2: Fix race conditions

The client-draw callback is running on the GL Thread, which will
be required to map the buffer. Map early, and pass the mapped
frame instead. On top of that, make sure to signal any pending
draw before trying to push EOS, as some pad locks might be taken.
This is the cost of using the same thread to control GStreamer and
to render GL.