dxapp: more input fiddling

- use directinput to get high resolution mouse deltas
- use MOVE messages to track position-in-window for picking
- sample input every frame, get rid of mouse() callback
- don't use PeekMessage() when inactive as we seem to stall and piss off
the GPU driver
hello: keyboard nav, zoom, scene scroll

- wasd move the noise around (regenerating the vertex data -- slow)
- rightclick + mousey zooms in/out
- mouse controls camera not model rotation now