Tools: replace coordinate reading function with CoordinateReader class

[dsizzle]

... this makes all the drawing tools use one standard CoordinateReader instead of several having their own. No user-visible change should occur; Airbrush, Freehand, Eraser, and Transparency should all behave as before.

I think this seems to fix #545 although I feel like I saw more brush-circle-where-drawing-starts artifacts. Maybe that's ok for this change because it'll be more easily fixable with standardized code?

[humdingerb]

I only see the brush-circle at the beginning of drawing with the Airbrush tool. But it id that before this change, too. Consolidating code is probably a good move in the right direction.

[dsizzle]

yay, merged