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