filters: cubic resampling: Cache coordinates mapping.
filters: cubic resampling: Cache coordinates mapping.

This moves the mapping computation out of the main loop (something I've
had in mind for quite some time) which yields in 4 x boost in speed.

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim