<flowRoot> (SVG 1.2 flowed text, which Inkscape writes for text typed
into a frame) was dropped without a word: usvg doesn't draw it. The
text pass now reads it as area type: the first <rect> of its
<flowRegion> is the frame (with the rect's transform), each <flowPara>
a paragraph (white space collapsed), styled through the same cascade
as <text>. Another kind of region leaves the text out with a warning.
Fixes#1568