Likely with the change to SDL's 'captured mouse' mode, mouse movements
during fullscreen became deltas *from the last movement*, not from
the last sampled position or frame. In order to correctly measure the
mouse movement, the deltas must be summed up and then reset when 'read'
at input-processing time.
Moving to summing the deltas threw many other mouse-related calibration
factors out of whack, so these factors are re-determined empirically
across a range of mice, from 400dpi 125hz to 30,000dpi 8khz. Reasonable
defaults are chosen targeting an 'average' mouse of 1000dpi, and the
mouse sensitivity precision is increased to allow finer user tuning.
