If users put their finger on the sensor between the bulge and
"un-bulge" area first and then swipe, the captured image would
be bad.
Skipping more frames can reduce the impact, so bump
ELAN_SKIP_LAST_FRAMES to 2.
Check for the mean calibration being outside of range to know whether we
require a recalibration. Continue with the usual checks if the
calibration value is within range.
The dimensions some sensors return is the maximum zero-based index
rather than the number of pixels. Assuming every sensor has an
even number of pixels is safe.
