Make xmp_set_position consistently clear pattern break/jump vars

[AliceLR]

Currently, xmp_set_position/xmp_next_position/et al. do not consistently clear the pattern break and jump variables in the player flow struct, meaning if these functions are called between this effect being used and the next row playing, the first row of the position set by xmp_set_position/etc. will play and then the pattern break or pattern jump will immediately take effect. Believe it or not, this was actually causing an audio bug in a MegaZeux game due to some unfortunate timing.

This patch simply makes these functions clear the relevant pattern flow variables whenever the player position is modified. It might also be necessary to clear f->loop_chn and f->rowdelay (not entirely sure).

[sezero]

No one else commented on this one. @AliceLR: this is well-tested, yes?

[AliceLR]

Yes. I need to look into whether or not f->loop_chn and f->rowdelay (or anything else) might need to be cleared but if necessary I can make another patch for those.

[sezero]

Yes.

OK then, merged.

I need to look into whether or not f->loop_chn and f->rowdelay (or anything else) might need to be cleared but if necessary I can make another patch for those.

Yes, another patch would be better