video: use new method to get QP table

This only matters for those who want to use vf_pp. The old API is marked
as deprecated, and doesn't work on Libav. It was broken on FFmpeg, but
has recently started working again - the fields in question were not un-
deprecated though. Instead you're supposed to use a new API, which does
exactly the same thing (what...?).

Also don't pass the QP table with mp_image_copy_attributes() - it
probably does more harm than it's useful.

By the way, with -vf=dlopen=TOOLS/vf_dlopen/showqscale.so, it appears
the table as output by recent FFmpeg is offset by 1 macroblock in X
direction and 2 macroblocks in Y direction, which most likely will
interfere with normal vf_pp operation. However, this is not my problem.

The only real reason for this commit is that we can finally get rid of
all libav* related deprecation warnings. (Though they are constantly
deprecating APIs, so this will not last long.)