Add support for ffmpeg's AVCodecContext::delay

[JoeSchiff]

This :type: int delay variable is currently inaccessible:

PyAV/include/libavcodec/avcodec.pxd

Line 192 in 8e32fe7

int delay

I assume this is supposed to be a reference to ffmpeg's AVCodecContext::delay.

This PR creates a property codec_context.delay which references ffmpeg's AVCodecContext::delay.

There is a separate, but similarly named, PyAV property codec.delay which is a reference to ffmpeg's AV_CODEC_CAP_DELAY.
We might want to rename one of these properties to prevent confusion.

Tests

Since the ffmpeg docs for AVCodecContext::delay state "Set by libavcodec.", I was unsure how to test this property. I ended up finding a file in the FATE suite with a detected delay value. I verified the value (312 for this example) using ffprobe.

If this is an acceptable testing strategy, then I could write up more tests to help merge some of the other PRs.

[WyattBlue]

Thanks! Looks good to me