Allow frequency changes on audio samples

[stevesims]

Based on work by @MovieVertigo in breakintoprogram/agon-vdp#143

extending to do some tidying up of audio system, and enhancing with a few new features

todo:

• [x] adjustable base sample rate for audio subsystem
• [x] configurable sample rate for sample data
• [x] adjustable base frequency for samples
• [x] sample looping control
• [x] streaming sample whole duration playback

[stevesims]

I've ticked the streaming sample whole duration playback... the solution for this requires explicitly setting the correct duration for playback using a new "set duration" command, which accepts a 24-bit duration value.

zero value duration for "whole sample" playback is problematic - allowing duration of note playback to be adjusted provides an alternate way to resolve this issue

it's still not an ideal streaming solution. ideally we should have a way to move from one buffer to another, and thus allow old/used buffers to be disposed of