Speculative fix for MaxTouchPoints returning incorrect value on Windows.
Windows 7 appears to sometimes return an incorrect value for max touch
points, returning 1 when there's in fact no touch devices present. This
breaks some web pages that use a non-zero number of touch points as a
signal that the devices has a touch screen. As this is specific to certain
Win7 configurations, I don't have a repro; however, reports in the bug
show that the SM_DIGITIZER setting does indeed return the correct status
of "no touchscreen". In light of this, we speculatively return 0 touches
if there's no touchscreen device present.