Fix h.264 frame/keyframe identification in the player.

If a NAL unit that determines a frame start appears as the last NAL
unit in the packet, then the frame start is not identified until the
start of the next packet.  If this new packet has its own NAL unit
that determines a frame start, and that NAL unit is not at the end of
the packet, then only one frame start would be counted rather than
two.  This is fixed by returning the total number of frame starts,
rather than a bool.

In theory, the same problem could exist in the recorder, but it
appears that the packets in the original recording may have some
trailing NAL units that are stripped by the ffmpeg libraries when
assembling the packet for the player, causing the situation not to
occur.