Remove dependency from swscale_internal.h to lavu/internal.h, it is no longer
needed for DECLARE_ALIGNED.
Remove dependency from swscale-example.c to swscale_internal.h by duplicating
the necessary code. The duplicated code is a hack and should be removed once a
cleaner pixel format information system exists. swscale-example.c is example
code on how to use the library and therefore shouldn't rely on internal