Let video-src unconditionally call _trigger_process() when it is
ready. This will either start the graph when it is the driver or emit
a RequestProcess event to the driver.
Let video-play-pull intercept the RequestProcess command and use it
to do a trigger_process(). Otherwise use a timer to pull in the next
frame.
It has the DRIVER flag set and sets a PW_KEY_PRIORITY_DRIVER value
to something higher than the source so that it becomes the driver.
Every timeout it does pw_stream_drive() to start the graph, which
will eventually call the process function with a pulled buffer to
display.
The framerate is set to something lower (80ms between frames) to
show the pull effect.
fixes#1484
