https://developer.gnome.org/glib/stable/glib-Testing.html#g-assert-cmpmem

I believe I'm happy with it now.

We expect the buffer to have no flags that the reference doesn't...
Meh. Makes MORX tests pass now. Need to better define the behavior.

Fixes https://github.com/harfbuzz/harfbuzz/issues/755

Fix build for subsetting code.

Interface for hb_subset, skeleton for the hb-subset cli, and basic testing rigging.

At any position, if state is not zero, mark unsafe-to-break before,
unless we can reason it safe.

At any position, if there's an action entry for end-of-text, mark
unsafe to break.

Also changes buffer diff impl to allow for flag differences as long
as the buffer glyph flags are superset of reference glyph flags.

With this, all MORX tests pass.