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.