dpb fills up with UsedForShortTermReference images

[mritalian]

In short when decoding, the dpb fills up with UsedForShortTermReference type images. These are never freed --ever, I can't even see in the code where the logic is that might free them when they are no longer needed (i.e. IDR frame).

I am using frame-parallel branch because master is incredibly slow in comparison.

Any help would be appreciated, thanks.