Fix bugs in component alpha combiners for separable PDF operators

Fix bugs in component alpha combiners for separable PDF operators

In general, the component alpha version of an operator is supposed to
do this:

       - multiply source with mask in all channels
       - multiply mask with source alpha in all channels
       - compute the regular operator in all channels using the
         mask value whenever source alpha is called for

The first two steps are usually accomplished with the function
combine_mask_ca(), but for operators where source alpha is not used,
such as SRC, ADD and OUT, the simpler function
combine_mask_value_ca(), which doesn't compute the new mask values,
can be used.

However, the PDF blend modes generally *do* make use of source alpha,
so they can't use combine_mask_value_ca() as they do now. They have to
use combine_mask_ca().

This patch fixes this in combine_multiply_ca() and the CA combiners
generated by PDF_SEPARABLE_BLEND_MODE.