edl: use start PTS of the source files as default start time
cache: be silent if no initial fill is requested

Hides the "Cache fill:" message with default settings.
video: fix display of cover art with vo_vdpau

vo_vdpau currently has a video queue larger than 1 entry, which causes
the video display code to never queue display the video frame. This is
because we consider cover art an endless stream of frames decoded from
the same source packet, and include special logic to actually only
decode and display 1 frame.

Also, make decode_image() also signal EOF in the cover art case.
player: suggest --no-audio-display on cover art