Round musical time to row boundary

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Pently has a few quirks because the length of a measure usually isn't an exact multiple of one tick. In "Happy Flappy Crappy" (bf98 in musicseq.pently), for example, it's 133.3 BPM at 3 rows per beat, which gets translated to 400 rows per minute, or 3606 / 400 = 9.015 NTSC ticks per row. pently_update_music accumulates the row's elapsed musical time in pently_tempoCounterHi, and at NTSC true speed (3606 rows per minute), musical time drifts in and out of phase with the tick on roughly a 10 second loop.

In the staccato portion of this piece, measures 17-25, the lengths of detached notes in patterns osti1 and osti2 drift back and forth between being shorter and being longer. The detached implementation relies on musical time per row to know when to cut the note. It also sounds different each loop because the piece's loop length (48 measures at 9 rows per measure, or 432 rows) isn't a multiple of a tick either (432 * 3606 / 400 = 3894.48 ticks).

Some NSF battle platforms (such as Famicompo Pico and Battle of the Bits) have expressed plans to detect song length and loop length by hashing the NSF player's emulated memory each frame and seeing when those hashes repeat. But if a piece's length isn't a multiple of a tick, variance in the fractional musical time will cause it to miss loops.

• [x] Characterize the behavior of detached by investigating how many ticks each note lasts at various points of the phase in this piece
• [ ] Subroutine to round current musical time to row boundary
• [ ] PENTLY_USE_TEMPO_ROUNDING_SEGNO
  Round at segno and dal segno
• [ ] PENTLY_USE_TEMPO_ROUNDING_PLAY_CH
  Round whenever a pattern begins on a particular track
• [ ] PENTLY_USE_TEMPO_ROUNDING_BEAT
  Round musical time to row boundary when pently_row_beat_part becomes 0 (requires PENTLY_USE_BPMMATH)

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The durations of detached notes in a 400 row per minute piece oscillate between 4 ticks (4/9 row) and 5 ticks (5/9 row). It wasn't noticed elsewhere because most other uses of detached have a row length closer to an even number of ticks.

• "K.231" (cover of Mozart's "Leck mich im Arsch") at 300 eighth notes per minute, or 12.0 ticks per row
• "The Naive Confidence" (aka "Twinkle Twinkle Little Star") at 270 eighth notes per minute, but because all notes on the instrument are multiples of 3 rows, it's heard as 40.1 ticks per 3 rows
• "Canon in D" at 450 thirty-second notes per minute, or 8.01 ticks per row
• "Tennis: Win Point" at 600 sixteenth notes per minute, or 6.01 ticks per row

Others have obvious decimals, more thoroughly mixing the short and long notes.

• "RHDE: Battle" at 468 eighth notes per minute, or 7.71 ticks per row
• "RHDE: Kalinka" at 560 sixteenth notes per minute, or 6.44 ticks per row