Fix Color Lines with a single color stop, and multiple stops all at the same stop offset

See also the comment here:
https://github.com/BlackFoundryCom/black-renderer/blob/9718f42dacb9ec9e13a2b3d95f2dc0cc0c0789fe/Lib/blackrenderer/font.py#L395-L419

Start by reading the comment and surrounding code in Lib/blackrenderer/font.py at lines 395-419. Trace how color lines handle a single color stop and multiple stops sharing one offset. Done means both cases are handled correctly without regressing the existing color-line behavior; no test file is named in the issue.

Written by the indexing model from the issue text.