DirectX9

Work on DX9 hasn't even started yet :). Feel free to use this space to organise & plan.

I would like to work on a DX9 backend as there are still plenty of people working with DX9, but right now it's not a priority. If no-one takes a crack at this then I will make a start on it, I just don't know exactly when that will be.

In terms of overall structure I wouldn't expect it to be wildly different to the DX11 backend, but since DX9 has a much larger API to target there will be a wider area to cover - not necessarily more complexity. Functionality that's exposed on individual interfaces like texture locking and such can just be implemented as a pass-through to the device or similar.