[OTLayout] Respect syllable boundaries for backtrack/lookahead matching

[OTLayout] Respect syllable boundaries for backtrack/lookahead matching

Originally we meant to match backtrack/lookahead across syllable
boundaries.  But a bug in the code meant that this was NOT done for
backtrack.  We "fixed" that in 2c7d0b6b80d412de3fddd443ed1a485ea1cbb03c,
but that broke Myanmar shaping.

We now believe that for Indic-like shapers (which is where syllables are
used), all basic shaping forms should be fully contained within their
syllables, so now we limit backtrack/lookahead matching to the syllable
too.  Unbreaks Myanmar.