examples: add a pull based video player

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