Implement wait_for_done for DpiTransfer

[EliteTK]

Motivations

I8080Transfer has this facility already, so this would introduce parity.

In general it can be useful when animating content on a display to avoid re-drawing more often than is necessary to ensure a smooth animation.

Solution

An async wait_for_done function on the DpiTransfer in the same style as the one on I8080Transfer.

Alternatives

I think it should be possible to achieve the same effect by simply implementing an appropriate DmaTxBuffer which tracks how much data has been transferred and synchronizes rendering to which frame is currently being requested combined with using Dpi::send with next_frame_en as true. But I think both styles would be appropriate to support.

Additional context

I've discussed this with @Dominaezzz on matrix (in the Embassy channel) and he recommended I submit this feature request.

I have looked into this and may consider contributing this but please don't hold your breath as I think the DmaTxBuffer route would personally be preferable at the moment.