Only tag, value, and global-ness of features are considered, not their
start/end offsets.

Such that we don't accidentally use info not in the cache key.

And include it in hb_shape_plan_t itself.

Fixes https://oss-fuzz.com/v2/testcase-detail/5754863779053568

Use normal, production, shared libraries.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1237

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11367

This is the root cause of bug worked around in 19e77e01.

Still no shape plan caching for variations though.

No other shaper will need shape_plan_data, by definition.  So, remove
abstraction layer and always create hb_ot_shape_plan_t as part of
hb_shape_plan_t.

This reverts commit 442a72d9.

Doesn't make sense.  No idea how my local compilers where happy with it!

Ouch :).

Although didn't need it...

Fix hb_blob_ptr_t::get_length () as well.

To remove custom null object next..