pdCanvas: fix cursor disappearing when Quick Fix tools are active

If a given tool doesn't care about the cursor, it will now default to
the standard arrow.
Quick-Fix tools: remove Saturation, add Clarity

Saturation is already covered by the far-superior Vibrance option, so no
loss there.  I mainly added it initially as I wanted to investigate
speed of HSL transforms when used as an on-canvas tool.

Clarity works by preferentially adjusting midtones, while leaving
shadows and highlights untouched.  Lightroom provides a similar option.
I like Clarity as a Quick-Fix tool as it's not only very useful, but it
can be implemented using LUTs, meaning I was able to merge Exposure,
Contrast, and Clarity adjustments into a very fast single-pass function.
Update README to reflect forthcoming 6.4 features