Some borders draw faint pencil-like artefacts

Describe the bug Some borders get slight artefacts at the outer edges and through the corners of the border, which almost look like faint pencil strokes. In the example below, it happens around the dark boxes, but not around the light boxes (at least not visibly so). Zoom in on the generated PDF to see it in action.

To Reproduce Reproduced in this REPL snippet

Expected behavior Those artefacts should not appear.

Screenshots

Desktop:

• OS: MacOS 10.13.6
• Browser N/A, but verified in the Preview.app PDF viewer and in Safari and Chrome's PDF viewers.
• React-pdf version v1.6.4