vd_ffmpeg: disable slice use that fails with size changes

vd_ffmpeg: disable slice use that fails with size changes

When not using direct rendering, vd_ffmpeg created an mp_image struct
before calling libavcodec decoder, so that possible slice support
could be checked from the mpi_image and output would be ready for
slice-drawing calls. However, this behavior is unsound with decoders
that can change output size, as the parameters can change after the
mp_image was created. Disable the code creating the mp_image at that
point, which also disables use of slices in this case. Slices are
disabled with threading anyway, so I think trying to add workarounds
to support them is not a high priority.

I think this code has always been buggy, but before common thread use
it was rarely executed because the direct-rendering case was used
instead.