VideoFrame format tweaks

f.pix_fmt -> f.format
f.to_rgba() -> f.to_rgb()
Docs should include all members
Notes and test framework
Add weak ref of Context to Stream

Then it can start encoding. There must be a nicer
way to do this...
Pipe library logs into Python logging
Fix log segfault: just use av_log_format_line
nogil all the things!
Remove duplicate Sw[sr]ContextProxy classes
Fix weakref callback

I thought `0` would be a C int, but it was a
Python int. Ergo, it was called when the weak ref
was broken, and an exception was triggered (and
promptly ignored).
Post log requests back to the main thread

Hopefully this does actually fix the threading
problem.
Start replacing av_log_format_line

It doesn't exist in Libav...
Log AVClass.item_name() instead of .class_name

This is the most reliable so far, and is what
FFmpeg/Libav do.
Merge branch 'logging' into develop