lavfi/showspectrum: fix outpicref initialization.

In 81f2549, output pixel format was changed from rgb24 to planar yuv,
but the initialization was left unchanged. As a result, the chroma
planes were left uninitalized. This was not noticed since the current
ff_get_video_buffer() seems to be initializing the buffer. This won't be
the case anymore after the Evil Plan.
lavu/frame: copy the channels field.
port FFmpeg lavc frame specificities to lavu functions.