[subset] Remove subset-glyf; want everything to point to new iter-based edition. Some of the code will resurface as impl builds out.

[subset] Remove subset-glyf; want everything to point to new iter-based edition. Some of the code will resurface as impl builds out.

Part of https://github.com/harfbuzz/harfbuzz/issues/1553

See comments.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1300

Fix hb_blob_ptr_t::get_length () as well.

Fixes all except for cmap.  To be done separately.

Part of https://github.com/harfbuzz/harfbuzz/issues/1146

It's true that blob->as<> should only be called on null or sanitized
data.  But this change is safe, so keep it.

Show up with gcc -O0.

There's a few more but those are functions that need to be filled in.

Maybe this is a lost battle...

There might be a better way to do this, but I couldn't find...