Steps through the input buffer 1 unicode character (glyph) at a time (ie, not necessarily 1 byte at a time).
Bitwise 'and' our this->mask with the mask for each glyph. This has the effect of limiting what charsets our this->mask can match.
| this |
charset structure |
| inbuf |
input text buffer (must be in UTF-8) |
| inlen |
input buffer length |