Initialize masks before mirroring

We were throwing away the rtlm feature mask set during
mirroring...
Add XXX note
Bug 72698 - Automatically support frac / numr / dnom

When seeing U+2044 FRACTION SLASH in the text, find decimal
digits (Unicode General Category Decimal_Number) around it,
and mark the pre-slash digits with 'numr' feature, the post-slash
digits with 'dnom' feature, and the whole sequence with 'frac'
feature.

This beautifully renders fractions with major Windows fonts,
and any other font that implements those features (numr/dnom is
enough for most fonts.)

Not the fastest way to do this, but good enough for a start.