Sometimes, when ScriptShape is presented with a surrogate pair which does
not map to a glyph, it returns two space glyphs instead of .notdef (0).
Detect this class of issues and handle appropriately.
git-svn-id: http://skia.googlecode.com/svn/trunk@11660 2bbb7eff-a529-9590-31e7-b0007b416f81