* imgproc: Prevent 1B overrun of 8C3 SIMD optimization

The fourth value read via v_load_q is essentially ignored,
but can cause trouble if it happens to cross page boundaries.

The final few iterations may attempt to read the most extreme
elements of S, which will read 1B beyond the array in most
aligment cases. Dynamically compute the stop. This could be
hoised from the loop, but will require a more extensive change.

Likewise, cleanup the iteration increment statements to make
it more obvious they do channel count (3) elements per pass.

This should resolve #16137

* imgproc(resize): extra check