Fix some jerkiness when playing back with no interpolation.

Fix some jerkiness when playing back with no interpolation.

When hitting a frame exactly, we'd choose [pts_of_previous_frame, pts]
as our candidate range, and always pick the lower. This would make us
jerk around a lot, which would be a particular problem with audio.

Fix by taking a two-pronged approach: First, if we hit a frame exactly,
just send [pts,pts]. Second, activate snapping even when interpolation
is off.

We still have a problem when playing back audio in the case of dropped
video frames (we'll output the same audio twice), but better audio
handling is somewhat outside the scope of this commit.