Make ringmod triggerwave based on mathematical frequency relationships

[acrouzet]

Idea 1:

• Make every triggerwave, except noise waveforms, a saw.
• Shift saw phase if triangle or tripulse is selected.
• Trigger on this falling slope.

[acrouzet]

The shifting could be done in the twsync handling with something like:

if (!twsync_cond_prenoise || (tw0_waveform & 0x8))
    {
        twsync_here = false;
        freq = tw0_freq;
        accumulator = (tw0_waveform & 0xb) == 0x1 ? tw0_accumulator + tw0_twshold : tw0_accumulator;
    }