Fix some signed overflow bugs

Fix some signed overflow bugs

In the macros for the PDF blend modes, two comp1_t variables are
multiplied together and then used as if the result were a
comp4_t. When comp1_t is a uint8_t, this is fine because they are
promoted to int, and the product of two uint8_ts fits in an
int. However, when comp1_t is uint16, the product does not necessarily
fit in an int, so casts are necessary.

Fix for bug 43906, reported by Siarhei Siamashka.