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.

This transfers ownership of the buffer instead of adding another reference.

Fix another memory leak.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/514

A newly allocated buffer doesn't need this call; it only adds overhead
for copying the data.

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