Add average qp to x264_image_properties_t

Hi,

Thanks for providing this project, it's super helpful.

I am a video researcher and working on codec performance. I found a bit of weird that x264 is not providing frame level QPs as a mean of analysis and only have f_crf_avg. So I add it to the x264_image_properties_t, close to where f_crf_avg lies in. Having access to frame level QPs is important when analyzing encoders and debugging rate control problems.

Thanks for viewing this PR.

Xu

Hi. In my opinion average QP is useless metric (it doesn't say anything about quality) so I don't see why it should be in public API. For research project you can modify libx264 source as you want but imho average QP doesn't worth it to be in public API.