[subset] Use hb_subset_input_t inside of subset_options_t so that input defaults are shared between the library and cli.

[subset] Update the subset fuzzer to determine which options to use based on data in the fuzzing test case.
Add support for toggling retain_gids.

* -Wshift-sign-overflow
* -Wmissing-prototypes

We might reintroduce it later, but for now remove, as it was unused.
Some things that should have been in this object (drop_hints, etc)
are already in hb_subset_input_t.  So, for now, keep everything there.

[subset] Have the subset fuzzer pull the text string to subset to from the end of the fuzzer provided data.