[coretext] Always compute log_clusters

To be used soon.
[coretext] Fix buffer resize handling

We can't really resize buffer and continue in this shaper as we are
using the scratch buffer for string_ref and log_cluster.  Restructure
shaper to retry from (almost) scratch.
[coretext] Use input clusters

Before, this shaper was returning UTF-16 cluster indices instead of
returning whatever cluster values the user had had set up in the buffer.
Ouch!
[coretext] Attach marks to base clusters

Fixes https://githu.com/behdad/harfbuzz/issues/49
to the extent that it can be fixed.