Correct off-by-one in 'mix' resize method.

Bryan Stillwell report. (fist) Doubles.
To 1.81.
Another fix to mix resize reported by Bryan Stillwell.
To 1.82.
Add a test to catch that.
Merge v1.82

* master:
  Add a test to catch that.
  To 1.82.
  Another fix to mix resize reported by Bryan Stillwell.
  To 1.81.
  Correct off-by-one in 'mix' resize method.
Simplified code a bit
Removed ability to make first pixel in a string lossy
More comments
Eliminated duplicate code in lossy search
Improved dithering