[coretext] Minor

It's hard to handle all possible NULL returns from CoreText.  Add one
more...
[coretext] Pass buffer direction to CoreText

Have to use a CTTypesetter for this.
Minor
API: Do not clear buffer-flags in hb_buffer_clear_contents()

After 763e5466c0a03a7c27020e1e2598e488612529a7, one doesn't
need to set flags for different pieces of text.  The flags now
are something the client sets up once, depending on how it
actually uses the buffer.  As such, don't clear it in
clear_contents().

Tests updated.