So it doesn't accidentally match our templates, etc.

Use rand=255 to mean "randomize".

Part of https://github.com/harfbuzz/harfbuzz/pull/803

Except if the set it to 1, which would mean "randomize"... Ugly.

Why does subset plan not have a hb_set_t of glyphs?

Ouch.

Add subset() call for GSUBGPOS struct and its dependencies.
Not hooked up anywhere.

We're going to (finally) use virtual methods for hb_serialize_context_t
customization, so don't need to carry a Serializer template variable
around...  Simplifies code.

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

Sorry for the noise, downstream custom builders.  Please adjust.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1107