https://bugs.freedesktop.org/show_bug.cgi?id=65544

Oops.  Thanks Khaled for catching this.

Not sure if it's useful, but it was missing.

We not namespace our types, so the hack is not needed anymore.

Add global shape_plan.  Unused so far.

Does not provide Uniscribe-compatible results, but should at least avoid
breaking hb-view due to out-of-order cluster values.

For RTL runs, ensure cluster values are non-increasing (instead of
non-decreasing).

Does not attempt to handle clusters in a Uniscribe- or HarfBuzz-compatible way;
just returns the original string indexes that CT maintains. These may even be
out-of-order in the case of reordrant glyphs.