swscale/input: Use more unsigned intermediates

swscale/input: Use more unsigned intermediates

Same principle as previous commit, with sufficiently huge rgb2yuv table
values this produces wrong results and undefined behavior.
The unsigned produces the same incorrect results. That is probably
ok as these cases with huge values seem not to occur in any real
use case.

Fixes: signed integer overflow
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ba209e3d5142fd31bb6c3e05c5b183118a278afc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>