Add ReportLab cubic path support

[eaton-lab]

Hi @tshead2, this is a simple feature addition I would love to have supported.

Summary

• add ReportLab support for SVG cubic path C commands and close-path Z / z
• factor SVG path command translation into a shared helper in the ReportLab renderer
• add a Behave feature that verifies the expected ReportLab path calls for move, line, cubic, and close commands

Details

Toyplot's ReportLab backend already handled simple SVG path commands, but it did not translate cubic Bezier
segments. This PR adds support for cubic path rendering so SVG paths that include C commands can be converted
into the corresponding ReportLab curveTo() calls.

The change also covers path closing, and adds a focused regression test that checks the backend issues the
expected beginPath(), moveTo(), lineTo(), curveTo(), close(), and drawPath() calls.

Testing

• python -m behave features/reportlab.feature --logging-level INFO --no-logcapture

Notes

• no public API changes
• scoped to ReportLab path rendering and its regression coverage

Example

Its useful for creating some cool plots like this and ensuring they can render to PDF.

[coveralls]

coverage: 94.508% (+0.005%) from 94.503%
when pulling cb1acd2 on eaton-lab:pdf_path_C_support
into d06e432 on sandialabs:main.

[tshead2]

Looks like a winner, many thanks!