add tempomult option to control playing speed

[yotam-medini]

This --tempomult option is very useful for practicing music.
Especially for choir singers.

[psi29a]

Can you explain how this would be used? Can you provide an example? :)

(aside from, speed it up :P )

[sezero]

BTW, This is an API + ABI change: Do we really want this?

[yotam-medini]

Say for example we download
https://www.cpdl.org/wiki/images/7/74/Ws-bwv-mp76.mid
and play the ~15 seconds segment 0:40-0:55 via
wildmidi -i 40 -j 55 Ws-bwv-mp76.mid
We can play it slower - for easy practicing - via (will take ~18 seonds)
wildmidi -i 40 -j 55 -T 0.8 Ws-bwv-mp76.mid
or faster - as the choir conductor's taste - via (will take ~8 seconds)
wildmidi -i 40 -j 55 -T 1.8 Ws-bwv-mp76.mid

I chose the '-T' option character as timidity has similar option:
-T n, --adjust-tempo=n
Adjust tempo to n%; 120 play MOD files with an NTSC Amiga's timing.

[yotam-medini]

About the API change.
An alternative would be to introduce global :(static): variable(s)
and add API functions to get/set them.

Also for both API-change or API-additions
it may be better instead of the 'float tempo_mult' value
to have something like

  struct _parse_cfg {
    float tempo_mult;
    // possible more future parameters
  };

with

  void init_parse_cfg(struct _parse_cfg *p) {
    p->tempo_mult = 1.0f;
    // possible more default future settings
  }

and in wildmidi.c have

  static _parse_cfg parse_cfg;
  ...
  int main(int argc, char **argv) {
    ...
    init_parse_cfg(&parse_cfg);
    ...
  }

and pass &parse_cfg instead of tempo_mult.
This will allow for future enhancements without API change.