dav1d shouldn't use fprintf(stderr) for logging, or if it must, such logs should be a build time option.

Embedders are unlikely to want logs emitted to stderr (certainly Chrome doesn't except in debug configurations). So there should either be an option to disable them at build time (preferable to avoid string bloat) or run time.

One way this could be done is to have all logs go through a dav1d_log "function" like:

#if defined(CONFIG_DAV1D_LOGGING)
#define dav1d_log fprintf
#else
#define dav1d_log(...)
#else
#endif

There are probably cleaner ways than this, but this ensures no binary bloat and avoids dealing with passing varargs.