volume overlay and menu timing/race issue

It when touching the volume rocker and changing views in the menu it is possible to get the volume overlay rendering to somehow interfere with the draw contexts, resulting in a flickering ui - that continues being possible to operate.

After further poking and triggering it again - when seeing the internal bowels of Otamatone. What happens is that we start DMA transferring the scratch-buffers in progress of rendering instead of the finished renderings.