AutoMapping: Removed the "touched layers" optimization

[bjorn]

The "AutoMap While Drawing" had an optimization where it skipped entire rule maps when none of their input layers had been "touched". This was based on the assumption that its rules would not need to be re-evaluated anyway.

Unfortunately, we can't make this assumption in general. The effect of certain rules might be relevant even if none of their input or output layers have been touched.

[eishiya]

Had a go with this using my enormous set of Automapping rules. None of the rules were affected negatively by the optimization. Removing the optimization has made Automap While Drawing even with a 1x1 brush noticeably more sluggish. It's still acceptable, but less pleasant than before, when it was practically instant.

I wonder if perhaps the optimization could be brought back, but with a rule map property like "NeverSkip" (previously discussed in #3313) that'll force that rule map to be used even if its layers weren't touched, so that those rules that are impacted negatively by the optimisation have some recourse.