Streaming sample playback enhancement

[stevesims]

It's currently technically possible to stream audio to the VDP for playback. This is done by sending the first few blocks of a sample across to a VDP buffer, marking that buffer as being a sample, kicking off playback of that sample, setting the desired complete playback duration, and then continuing to send over blocks of the sample. So long as the z80 keeps up sending data across to the VDP in principle the sample playback should work and play to the end. (if it doesn't keep up, playback will loop, which will be slightly odd)

The down-side to this approach is that this will eventually fill the VDPs memory.

A proposed solution to this would be to mark a sample as "disposable" - and to throw away memory blocks once they have been played.

Support for this would likely require the buffered command API to allow the deleting of individual blocks. When playing back samples of this proposed format, on reaching the end of a block sample playback system would just delete that block (which would always be at block index zero).