avcodec/hap: add "compressor" option to Hap encoder to disable secondary compression

The secondary compression in Hap is optional, this change exposes that option to
the user as some use-cases favour higher bitrate files to reduce workload
decoding.
Adds "none" or "snappy" as options for "compressor". Selecting "none" disregards
"chunks" option: chunking is only of benefit decompressing Snappy.

Reviewed-by: Martin Vignali <martin.vignali@gmail.com>
Signed-off-by: Tom Butterworth <bangnoise@gmail.com>
