fix seek() with no previous frame displayed allow stepping forwards through a video source frame by frame
fix seek() with no previous frame displayed
allow stepping forwards through a video source frame by frame

--HG--
extra : convert_revision : svn%3A14d46d22-621c-0410-bb3d-6f67920f7d95/trunk%402522

infowantstobeseen/pyglet-darwincore,infowantstobeseen/pyglet-darwincore,infowantstobeseen/pyglet-darwincore,infowantstobeseen/pyglet-darwincore,infowantstobeseen/pyglet-darwincore