SConstruct: Unify the cprocess and process builds

cprocess and process don't have to be built separately anymore now that
Python 3 allows compiling with the aliasing optimizations on.
Let the subsequent build libraries use the process environment

I don't know if there's an easy way to reduce the number of libraries the
external stuff needs to link to.
Add the first quality hint parameter

Coded image codecs will get this in the function call, but video and audio
frames will probably pass the hint in the structure.
Add audio_mix_overwrite function

Handy for combining two potentially-disjoint audio frames.
Fix a G_LOG_DOMAIN or two
FIX: Composite order on GL workspace was backwards

I'll have to revisit this when I add compositing modes.
FIX: logging with one string parameter should work, too
FIX: raw_timecode not declared

I forgot where I put it, but clearly it's not needed anymore.
matroska: FIX: Silence unavailable samples

Samples outside the returned frame's range come back as None in Python;
here we substitute zero for the missing samples.
libdv audio decoder support

Somehow I committed the SConstruct changes before the actual module, but
here it is, support for libdv's audio decoder. I got most of the way to
working video decoder support when I found out that libdv spits out all
video as 4:2:2-- with chroma samples *doubled* if necessary. Completely
wrong. I may go back some day and fork it with proper 4:1:1 support, but
for now, libav's video decoder works, and libdv's audio decoder handles
errors much better. The combination should be fine.
cprocess: FIX: audio_overwrite_frame did not handle empty output frames correctly