Previously this was hardcoded to 2 pixels, which is too low for tablets and
not taking into account DPI. Now we set it equal to the tweak threshold, so
you either always do click or drag.
The default distance of 10 pixels may be quite far for something to be
considered a click, and we'll need to see how well it works. But I find this
to help a lot when selecting vertices in quick succession.
Thanks to Julien for spotting this.
e815784aa6