This functions more or less identically to using a media out value, but the
desired speed is preserved through block length changes, even if the block's
length is reduced to zero (i.e. no rounding errors).
With differently set speeds, these functions would mess with the timing in ways
that would be confusing to the user. Now they act in a much more intuitive way
and also prevent negative media times.
An earlier commit implementing media in and out parameters was primarily for
this addition. A simple control dialog for the clip's speed presentation
reimplemented from the old codebase.
itsmattkc