MMX removal, part 1

This PR removes MMX/ISSE code from most internal filters with the following reasons:
* User base with pre-SSE2 CPUs is nonexistent. We've been compiling official binaries with /arch:SSE2 and no one ever complained. Not to mention encoding on these machines would be painfully slow regardless of MMX presence in the core.
* With additional colorspaces introduced, MMX code will very soon become outdated and no one is willing to update it.

Not touched:
* RGB24->RGB32 conversion - doesn't have SSE2 version, only C and SSSE3. I did remove MMX from RGB32->24 conversion because C code was actually faster than MMX.
* Compare filter - SSE2 requires clip to be mod-16 and that isn't so easy to satisfy. This restriction will most likely be removed later during full refactoring.
* Overlay and resample - innocenat will handle those.

The only possible drawback of these changes on SSE2 CPUs is processing of unaligned frames, that now will dispatch to C version. This should not matter as the only way of getting unaligned data is using unaligned crop, which is not the default and is strongly discouraged.