Add fill tool support on strokes

[seam0s-dev]

This PR adds support for fill on strokes while using the fill tool as requested in #2615.

[seam0s-dev]

Test cases:
stroke fill pattern.graphite.txt

[Keavon]

See the video for the issues this encounters:

capture_10_.mp4

Here's the file with this shape (remove the .txt from the file after downloading it):

issues.graphite.txt

[seam0s-dev]

@Keavon The issue does not seem to affect shapes constructed using pen tool:

ellipse_vs_path_shapes_fill_overlay.mp4

[Keavon]

You likely need to take into account the local transform, the nested transform spaces to reach from local space to document space, and the stroke transform (which is affected by scaling or skewing a grouped layer, for example).

[Keavon]

I have force pushed this branch to move it to a new base. Ensure you have the branch checked out. Then please run git stash on any local changes (and ask me before following these steps if you have any un-pushed commits on your local machine). Then fetch this branch and git reset --hard origin/2615-fill-tool-on-strokes. And finally git stash pop your stashed local changes, if you stashed them.