Make the Fill tool work on strokes

[Keavon]

When hovering over a layer stroke, it should display the dotted overlay like we do for fills:

So in this image, we'd have the white dotted lines over the black layer stroke instead. See https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/strokeStyle.

Additionally, we'd like to prevent the dotted overlays from extending into half the stroke area, like we see above. This can be achieved by using some compositing tricks with the 2D canvas API to render the stroke in such a manner that it erases the drawable area where the fill dotted pattern goes. (This can't affect other overlays also drawn in that area, though, so no erasing the full canvas in that area.)