Assuming master->last.valuators is in screen coords, SD's are always in device
coordinates.
1. If an event comes in, scale masters->last to the device, drop into device's
last->valuators.
2. Apply motion from the actual event
3. Scale back to screen coords, check if we may need to cross screens
4. Drop screen coords into master->last
5. Rescale to device coords, drop into deviceValuator event and SD->last
6. Drop screen coords into ev->root_x/y
Whoopee...
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