Switch regression tests to swscale.
Plain C, x86-32 and -64 have been tested and should work, other
archs that had asm optmizations in swscale likely will need some fixes
to either fall back on C if SWS_BITEXACT is set or make the asm match C.
This also disables the PAL8 test as neither swscale nor the old scaler
really support PAL8 output, imgconvert supported a fixed 666 palette
as output and swscale supports fixed 884 and 422.
Originally committed as revision 15305 to svn://svn.ffmpeg.org/ffmpeg/trunk