Bug: Outline files are completely black -- Blocking Release

[kschelonka]

Outline files are black images when they should be a black outline on transparent background, with black scale bar in the lower left corner.

Bug introduced after #102.

Example manifest with bad outlines: s3://prod.slapp.alleninstitute.org/trial/seg_run_id_966/20200518134310/manifest.json

Bug affects both the full outline and the 128x128 pixel images.

Results in this behavior on the tool: https://files.slack.com/files-pri/TLEGP6YLA-F013VD653PC/peek_2020-05-18_13-57.gif

Fix needs to:

• [x] Produce the new files and demonstrate that they have appropriate transparency (can be done in a SageMaker GT app)
• [x] Fix test cases on test_image_utils.py. ~The expected arrays are not in the proper format (should contain an alpha channel).~ New implementation does not require alpha channel, but still needs to update expected test case.

[kschelonka]

I'm going to take a look and see where we may be missing test coverage. @djkapner will work on debugging.