Add support for units in `UniqueCoords`, `Rasterize`, and `Potrace` transforms #47

Job	Run time
Julia 1.9 - ubuntu-latest - x64 - pull_request	6m 19s
Julia 1.9 - macos-latest - x64 - pull_request	6m 35s
Julia 1.9 - windows-latest - x64 - pull_request	8m 0s
Julia 1 - ubuntu-latest - x64 - pull_request	5m 58s
Julia 1 - macos-latest - x64 - pull_request	6m 30s
Julia 1 - windows-latest - x64 - pull_request	7m 29s
	40m 51s

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