Add way to change the loop count of an existing track

[edo9300]

Both in the previous api and the new one, there is no clean way to change the looping state of a track after it's been started.
With the previous api, while dealing with a "Music" track I had resorted to pausing the track, getting its current position, then playing it with the looping flag changed, and immediately setting the position to the one it was currently at before.
It would be nice to have a dedicated function to apply this behavior and since the api got completely redesigned, it might be worth to add it before the first release in case.

[icculus]

Do we just want something like this?

/**
 * Change the number of times a currently-playing track will loop.
 *
 * This replaces any previously-set remaining loops.
 * A value of 1 will loop to the start one time. Zero
 * will not loop at all. A value of -1 requests infinite 
 * loops. If the input  is not seekable and this value
 * isn't zero, this function will report success but the
 * track will stop at the point it should loop.
 * 
 * The new loop count replaces any previous state,
 * even if the track has already looped.
 * 
 * Setting the loop count to -1 will cause the track to
 * loop infinitely.
 *
 * This has no effect on a track that isn't currently playing.
 * Stopped tracks can specify a loop count while starting via
 * MIX_PROP_PLAY_LOOPS_NUMBER. This function alters
 * that count in the middle of playback.
 */
bool MIX_SetTrackLoops(MIX_Track *track, int num_loops);

[edo9300]

That would be good