Consider gluing extra bytes onto fuzz_skrifa_outline corpus to use to populate OutlineRequest

[rsheeter]

Ref #973. Currently we just take a cross of likely options, the fuzzer can't control the specific options via the incoming data.

Naively it would be nice if the fuzz_target could have multiple inputs, such as corpus + additional Arbitrary (rust-fuzz/libfuzzer#119).

[rsheeter]

Per @garretrieger HarfBuzz just takes the end of the file as options. This probably also works, relying on the fuzzer to generate extra bytes. My inclination is to glue more on regardless, just to minimize interference with the bytes that are likely to "look like" a font.