Since AVFilterContext are freed automatically, this wrapper class
serves no purpose.  Let's remove it.

If the FFmpeg filter outputs planar data, interleave it, just like the
FFmpeg decoder plugin does.

The two were never used at the same time, and merging them saves one allocation.

av_frame_get_buffer() leaks memory if buffers were already allocated.

Fixes one of the memory leaks of https://github.com/MusicPlayerDaemon/MPD/issues/514