Buffer audio instead of firing an interrupt at 22,050Hz

[Gadgetoid]

As brought up by @zenodante in #562 - firing an audio callback at 22kHz will ransack the instruction cache among other things. We should be feeding a ~441 sample buffer at ~50Hz instead.

[Daft-Freak]

Checking if the DAC should be enabled every sample probably isn't great either... https://github.com/pimoroni/32blit-beta/blob/master/32blit-stm32/Src/sound.cpp#L22

[Gadgetoid]

I did some sleuthing for the old audio code that used DMA and a buffer, and recalled that I never did manage to get it working. The system would just lock up solid and I had to abandon all hope in favour of... this travesty. So... yep. Long overdue a fix.

[zenodante]

I will work on it, after I receive the hardware. Current my h750-dk boards don't have a DAC audio setup. (Q_Q)/