stroke: Fix large line widths for fallback stroke shaper

Fix the test case line-width-tolerance for the fallback stroke shaper.
Instead of drawing quads between spline points, draw triangle based on
the actual spline edges. This roughly follows the approach of the
tristrip and polygonal shapers.

This commit also replaces the fixed point
_lines_intersect_or_are_coincident with slower, floating point
intersection code.  _lines_intersect_or_are_coincident was faultily
trying to skip doing an actual division.
stroke: Use round-joins near inflection points of splines

Similar to b7bd5ae4f3da44131261711bb236cd7aa24a3ae3, but applied to the
fallback stroke shaper.