captouch: improved signal processing

important: unfortunately we're still operating with once-at-boot calibration policy. if you hold the badge wrong during boot the driver does not stand a chance, please follow the calibration instructions in the troubleshooting guide when evaluating the performance. recalibration in captouch demo is also possible at this point.

there's a bunch of constants in st3m_captouch.c to play around with. while it is tempting to set thresholds very low, this in our experience messes with backtouch side effects, so we'd suggest decrementing them in small steps if needed - we got there by incrementing them a little whenever we saw issues.

also this modification shows slightly different backtouch interference behavior due to the high gains applied to small signal sums. we personally find this to be acceptable.