Clear only dirty areas of a canvas.

[chenba]

Fixes #4463

[codecov-io]

Codecov Report

Merging #4659 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4659      +/-   ##
==========================================
- Coverage   42.18%   42.13%   -0.05%     
==========================================
  Files          43       43              
  Lines        1880     1882       +2     
  Branches      348      348              
==========================================
  Hits          793      793              
- Misses       1087     1089       +2

Impacted Files	Coverage Δ
server/src/pages/shot/pen-tool.js	25% <0%> (ø)	⬆️
server/src/pages/shot/drawing-tool.js	10.14% <0%> (-0.31%)	⬇️
server/src/pages/shot/highlighter-tool.js	16.94% <0%> (ø)	⬆️

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[jaredhirsch]

Hmm. Seeing some weird behavior here, probably related to rounding off selection areas when undoing / redoing drawings. I'll add some screenshots.

[jaredhirsch]

OK, start with an image plucked from mozilla.org:

Highlight across part of the image:

Undo and redo a few times. Note the blurriness and offset of the chunk that was highlighted - this only gets worse as you undo/redo some more, or undo/redo on top of additional drawing areas:

As I mentioned in the review, I'm guessing this comes from rounding off subpixel distances, but maybe it also has to do with pixel density issues?

[jaredhirsch]

Here's how it looks after undo/redo about 20x, ending with the highlight reapplied:

[chenba]

So strange that it's not reproducible on every shot. Even with two shots of identical dimensions, I'd get different results. 😕

[chenba]

The issue with the blurred area is not from this PR and is now tracked in #4678.