[Cin] "workaround_bugs=1" for ffmpeg decode

I run into some cool but unwanted image distortion while playing back and forth sample file ~/matrixbench_highdivx_ac3.avi from samples.mplayerhq.hu sadly, setting this avctx flag doesn't fix artefacts, but I send it out anyway because someone might run into buggy early encode requiring this workaround variable to be set... I think you also can set it via opts file, but not tested this (because for me workaround does not improve image quality after few fast forward/fast backwards)