Fix handling of unknown characters in SVG output. (mathjax/MathJax#3224)

[dpvc]

This PR fixes an issue with the handling of the width of characters that are not in the MathJax fonts in SVG mode that could lead to overlapping characters. The SVG wrapper's toSVG() code has been reordered to make this work properly. The handling of unknown characters has been moved to the top, and if-then-else has been broken into separate if-then with returns, with the width of previous unknown characters being taken properly into account (which was what wasn't being done).

This only affects text that is all in the same MathML node, so you can test using \text{aaa 中文 bbbb} for the unknown and p sections of the code. To get a c test, you need to use the mathjax-tex font with \text{aaa\U{220C}bbb}.

Resolves issue mathjax/MathJax#3224.