Implement `lavfi-tempo` audio filter, which uses `atempo` or `ascale`
form libavfilter for setting the tempo.
Users (myself included) seem to be always looking for alternative tempo
scalers to try, and this provides a faster implementation (or two), and
may prove useful beyond performance/speed.
`atempo` is picked as a default, and as a fall-back if `ascale` is not
available.
Signed-off-by: Mohammad AlSaleh <[email protected]>
Mohammad AlSaleh
dc8101574d