Incorrect tiling pattern rendering when step smaller than bbox

From a remark by Rob Wu in a PDF.js project PR, I noticed that we're not doing tiling patterns correctly when the XStep or YStep is smaller than bounding box width / height. We're doing exactly what he told not to do.

Base pattern:
\/

Expected rendering of repeated pattern (recall xstep = half of width):
\XXXX

Actual rendering if truncating pattern width to xstep:
\\\\\