Add support for Latin-1, Cyrillic, and CJK characters inside \text{} (#508)

Summary:
This diff provides support for Latin-1, Cyrillic, and CJK characters
inside \text{} groups.  For Latin-1 and Cyrillic characters we use
glyph metrics from a glyph from Basic Latin that has roughly the same
bounding box.  We use the metrics for a capital 'M' to approximate the
full-width CJK characters.  Half-width characters are not supported yet.

Test Plan:
- make test
- make screenshots

Reviewers: emily

test/screenshotter/ss_data.yaml

@@ -114,6 +114,7 @@ Symbols1: |
     \maltese\degree\pounds\$
     \text{\maltese\degree}
 Text: \frac{a}{b}\text{c~ {ab} \ e}+fg
+Unicode: \begin{matrix}\text{ÀàÇçÉéÏïÖöÛû} \\ \text{БГДЖЗЙЛФЦШЫЮЯ} \\ \text{여보세요} \\ \text{私はバナナです} \end{matrix}
 UnsupportedCmds:
     tex: \err\,\frac\fracerr3\,2^\superr_\suberr\,\sqrt\sqrterr
     noThrow: 1