doxy: Document avlog

Provide some information for every function and add a group.
x86: xviddct: Employ more specific ifdefs

This avoids building mmxext and sse2 code when disabled by configure.
x86: fdct: Initialize optimized fdct implementations in the standard way
swscale: provide a default scaler if none is set

Lanczos for general case, sinc for upscaling, Gaussian for
downscaling. According to current literature these scalers
should be the best quality-wise algorithms for each case.

Inspired from a patch by wm4 <nfxjfg@googlemail.com>

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
x86: dsputil: Separate ff_add_hfyu_median_prediction_cmov from dsputil_mmx

The function does not depend on MMX and compilation without MMX enabled
fails if the function is compiled conditional on MMX availability.
oggparsevorbis: return meaningful errors

Signed-off-by: Diego Biurrun <diego@biurrun.de>