Fix handling of input if not in native endianness, and add support for
9/10-bit output. This allows us to force endianness of YUV420P 9/10bit
in the H264/10bit fate tests, which should fix them on big-endian
systems.
PPC and x86 code is split off from swscale_template.c. Lots of code is
still duplicated and should be removed later.
Again uniformize the init system to be more similar to the dsputil one.
Unset h*scale_fast in the x86 init in order to make the output
consistent with the previous status. Thanks to Josh for spotting it.
