front: Test mask colors in RGB 3-space.

The previous algorithm would enormously penalize errors in the red and
green channels:
 - red channel errors were magnified by 2^16 in the distance metric
 - green channels errors were magnified by 2^8 in the distance metric

This algorithm gets the independent RGB components and computes the
distance metric as the square of the pythagorean distance in cartesian
3-space.
(cherry picked from commit f99a8b9a98aef094bbc89fd05d5fa3a5ab3bad3a)
front: Match touch mask colors only if they fall within a tolerance.

Along with commit f99a8b9a98aef094bbc89fd05d5fa3a5ab3bad3a, this
resolves Issue #385.
(cherry picked from commit 76fb4cea29f3f2382752b95fa40093bcf18c8289)
doc: Update changelog.