processing. This fixes several problems:
* Double tap and drag now works. (Problem reported by Andrew Pimlott
  <andrew@pimlott.net>.)
* Locked drags are handled correctly, even for button 2 and 3
  tap-and-drag operations.
* The [LR][TB]CornerButton and TapButton[123] parameters now work also
  for button values > 3.
* The old code was too hard to understand.