fate: Split fate-pixdesc tests and dispatch them through Make

This allows running all the tests individually and/or in parallel.
ppc: idctdsp: Immediately return if no AltiVec is available

This is how all the other init functions operate.
idct: Split off Xvid IDCT

The Xvid IDCT is only required to decode some Xvid-encoded MPEG-4 files,
so there is no point in having it as an unconditional part of idctdsp.
pngenc: Drop pointless pointer cast in png_write_row()
tscc: Eliminate pointless variable indirections in decode_frame()
Revert "vf_interlace: deprecate lowpass option"

This reverts commit 35b05c5184fb3aa1191e2d1e7f1cae1e11f344a4.
A warning is introduced in case this option is used.
pngdec: correctly indent macros
pixdesc: K&R formatting cosmetics

Also change some comments to multiline.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
h264: prevent theoretical infinite loop in SEI parsing

Properly address CVE-2011-3946 and parse bitstream as described in the spec.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
flv: Index the audio stream

And leverage the video index if the video is just disabled as wm4
did in an initial patch.
af_join: Set the output frame format
af_channelmap: Set the frame channel layout

Otherwise the frame would show the first layout matching the
channel count.
ppc: fft: Build AltiVec optimizations in the standard way