Add support for textLength and lengthAdjust. The support for
lengthAdjust is actually not to the SVG standard, because the value it
supports is a nonstandard "size" option, i.e. lengthAdjust="size". As
it's non-standard, it's prefixed tpsvg, i.e. tpsvg:lengthAdjust="size".
This attribute causes the text's size to be adjusted such that the
string will fit within the value specified by the textLength attribute.