Avoid unnecessary vsnprintf() calls in log_callback()
Avoid unnecessary vsnprintf() calls in log_callback()

mov_read_trak() and mov_read_trun() generate a lot of AV_LOG_TRACE
logs, which are ultimately thrown away. However, log_callback() still
formats and processes them. This causes av_log() to - at least in some
cases I observed with perf record - to take 97% of
avformat_open_input() run time. Improve this by returning from
log_callback() early when a log is obviously not needed.

PyAV-Org/PyAV,PyAV-Org/PyAV