It no longer gets stuck in a loop so easily.

The key thing I realized was that there is a fundamental difference to
how the ATSUI and CoreText APIs handle text buffers and the start
offsets into them:

ATSUCreateTextLayoutWithTextPtr() takes a pointer to the whole buffer,
plus start offset and length. These are stored as part of the layout
object. Other ATSUI calls that use the layout object interpret index
parameters as relative to the buffer start, not relative to the start
offset of the layout object.

CTTypesetterCreateWithAttributedString() on the other hand takes an
attributed string that has been created from a CFString that already
is based on a potential substring of a buffer. I.e. the CTTypesetter
has no knowledge of the original buffer. All index parameters are
relative to that substring of the buffer, as are return values.

Change-Id: I7026f323d90a72e1ae1784a69f521e347baffaea