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()