fix: consume stderr on wait

[nathanbabcock]

Fixes #70

The stderr channel of the FFmpeg process is configured with Stdio::piped() by default, which is used to parse log messages for the iterator. However, this can cause problems when used with FfmpegChild.wait(). Log messages will continue to fill up that piped stdio channel without ever being consumed. If there aren't too many log messages, the command will finish successfully. There's enough room for several hundred log lines, which is why this bug never surfaced before. However, once the whole buffer is filled, then FFmpeg will hang while it waits for more room to insert log messages (which continues indefinitely).

The solution is to automatically drop consume the piped stderr channel when wait() is called.

[nathanbabcock]

One problem with this solution revealed by the CI builds: on Unix, dropping stderr will send a SIGPIPE signal and FFmpeg will immediately exit.

The drop() solution will work as a stopgap on Windows, but it seems like a full solution will require:

• Adding the stderr(), stdin(), and stdout() methods to FfmpegCommand to parallel std::Command and allow full control over configuring the IO channels
• Contextual error messages if, after spawning the FfmpegChild, wait() is called in combination with piped stderr, or iter() is called without piped stderr
• Keep the defaults backwards-compatible (i.e. all channels piped by default, since iter() is the primary use case)