Fix path coordinate mapping for clockwise Arcs

The Arc class's to_path_coordinates method assumed that all Arcs were
counter-clockwise.  This CL corrects the mapping for clockwise arcs.

Also add unittests.

BUG=b:148627899
TEST=unittests

Change-Id: Idc32c8278ded932a9c9485df962caad039c038d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/optofidelity_TPPT_analysis/+/3059461
Tested-by: Sean O'Brien <seobrien@chromium.org>
Reviewed-by: Harry Cutts <hcutts@chromium.org>
Commit-Queue: Sean O'Brien <seobrien@chromium.org>
2 files changed