test-picking: Don't manually paint the stage

test-picking: Don't manually paint the stage

Instead of using an idle handler that synchonously paints the stage
by manually calling clutter_actor_paint (stage) the test now uses an
idle handler that repeatedly queues a redraw on the stage.

do_events is now called from a "paint" signal handler for the stage, and
instead of manually determining the fps the test now uses
CLUTTER_SHOW_FPS=1 instead.