Add separate parsing of fatal logs

[avsaase]

This PR adds a separate parsing of fatal error logs so that they can be handled separately. I initially did this by adding a FfmpegEvent::LogFatal but in the last commit I changed this to make it a single FfmpegEvent::Log with a LogLevel field. I don't have a strong opinion in either direction so I'll leave the decision up to you.

I also changed the log parsing logic to check if a log line contains the log level in square braces (instead of starting with it) because I encountered a few logs like this that would otherwise be parsed incorrectly:

[swscaler @ 000001ceadc00b80] [swscaler @ 000001ceadd3b7c0] [warning] deprecated pixel format 
used, make sure you did set range correctly

I didn't add tests because there currently aren't any for the parse_next_event function and I didn't know how you want that set up.

[nathanbabcock]

@avsaase I will take a closer look within the next few days, but at first glance this looks awesome. I especially like that it handles the edge case when log level is not the first item on the line.

Thanks for taking the time to contribute 🙏