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

theblacklion/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,mammadori/pyglet,oktayacikalin/pyglet,mammadori/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,mammadori/pyglet,mammadori/pyglet,oktayacikalin/pyglet,theblacklion/pyglet