Fix placement of tspan in RTL text blocks

It turns out that the text layout code did not consider RTL at all when laying out individual spans within chunks. Now it does that, and it fixes text-anchor for multi-span chunks.

I've put in a test reference file, but it is pretty terrible. The SVG has some Persian text, but I think the Roboto fonts used for the test suite don't have those glyphs - so the test reference PNG has Unicode hex boxes. At least it shows the correct alignment.

Fixes #791 (closed)

Fixes #789 (closed)