Blender Git Loki

Improve proxy building performance

====Principle of operation====
Proxy rebuild job will spawn 2 threads that are responsible for reading
packets from source file and writing transcoded packets to output file.
This is done by functions `index_ffmpeg_read_packets()` and
`index_ffmpeg_write_frames()`. These threads work rather quickly and
don't use too much CPU resources.
Transcoding of read packets is done in thread pool by function
`index_ffmpeg_transcode_packets()`.

This scheme is used because transcoded packets must be read and written
in order as if they were transcoded in one single loop. Transcoding can
happen relatively (see next paragraph) asynchronously.

Because decoding must always start on I-frame, each GOP is fed to
transcoding thread as whole. Some files may not have enough GOPs to be
fed to all threads. In such case, performance gain won't be as great.
This is relatively rare case though.

According to FFmpeg docs some packets may contain multiple frames, but
in such case only first frame is decoded. I am not sure if this is
limitation of FFmpeg, or it is possible to decode these frames, but in
previous proxy building implementation such case wasn't handled either.

Similar as above, there is assumption that decoding any number of packets
in GOP chunk will produce same number of output packets. This must be
always true, otherwise we couldn't map proxy frames to original perfectly.
Therefore it should be possible to increment input and output packet
containers independently and one of them can be manipulated "blindly".
For example sometimes decoded frames lag after packets in 1 or 2 or
more steps, sometimes these are output immediately. It depends on
codec. But number of packets fed to decoder must match number of
frames received.

Transcoding contexts are allocated only when building process starts.
This is because these contexts use lot of RAM. `avcodec_copy_context()`
is used to allocate input and output codec contexts. These have to be
unique for each thread. Sws_context also needs to be unique but it is
not copied, because it is needed only for transcoding.

====Job coordination====
In case that output file can not be written to disk fast enough,
transcoded packets will accumulate in RAM potentially filling it up
completely. This isn't such problem on SSD, but on HDD it can easily
happen. Therefore packets are read in sync with packets written with
lookahead.
When building all 4 sizes for 1080p movie, writing speed averages at
80MBps.

During operation packets are read in advance. Lookahead is number of
GOPs to read ahead. This is because all transcoding threads must have
packets to decode and each thread is working on whole GOP chunk.

Jobs are suspended when needed using thread conditions and wake signals
Threads will suspend on their own and are resumed in ring scheme:

```
read_packets -> transcode -> write_packets
^ |
|______________________________|
```
In addition, when any of threads above are done or cancelled, they will
resume building job to free data and finish building process.

====Performance====
On my machine (16 cores) building process is about 9x faster.
Before I have introduced job coordination, transcoding was 14x faster.
So there is still some room for optimization, perhaps wakeup frequency
is too high or threads are put to sleep unnecessarily.

---------

====Code layout====
I am using `FFmpegIndexBuilderContext` as "root" context for storing contexts.
Transcode job is wrapped in `TranscodeJob` because I need to pass thread number, that determines on which GOP chunks this job will work on.
`output_packet_wrap` and `source_packet_wrap` wrap `AVPacket` with some additional information like GOP chunk number (currently `i_frame_segment`).
These 2 structs could be consolidated which will simplify some auxilary logic. This is bit tricky part because sometimes `output_packet_wrap` must
lag one step after `source_packet_wrap` and this needs to be managed when jumping between GOP chunks properly.

Other than that, I am not super happy with amount of code and mere setup this patch adds. But it doesn't look like anything could be simplified significantly.

====Problems / TODO====
I am not aware of any bugs currently

Maniphest Tasks: T85469

Differential Revision: https://developer.blender.org/D10394