avconv: use different variables for decoded and filtered frame.

Makes the code less obfuscated and fixes encoding one video stream to
several outputs.

Also use avcodec_alloc_frame() instead of allocating AVFrame on stack.

Breaks me_threshold in avconv, as motion vectors aren't passed through
lavfi. They could be copied manually, but I don't think this misfeature
is useful enough to justify ugly hacks.
