The final edge clamp computes the source position using int
multiplication before shifting. With sufficiently wide inputs this
overflows, which may suppress the clamp and leave the last output
pixels interpolated with the padding byte.
Promote the multiplication to int64_t in the C, MMXEXT and VSX
implementations.
Add a regression test covering the rightmost pixel of a wide upscale,
which is wrong before this change on both the C and the MMXEXT path.
Fixes: signed integer overflow: 15 * 255918080 cannot be represented in type 'int'
Fixes: #21591
Signed-off-by: iSold Leo <s@qwqlog.com>
ec2a4105e2