Eliminate mask_bits from all the scanline fetchers.

Eliminate mask_bits from all the scanline fetchers.

Back in the day, the mask_bits argument was used to distinguish
between masks used for component alpha (where it was 0xffffffff) and
masks for unified alpha (where it was 0xff000000). In this way, the
fetchers could check if just the alpha channel was 0 and in that case
avoid fetching the source.

However, we haven't actually used it like that for a long time; it is
currently always either 0xffffffff or 0 (if the mask is NULL). It also
doesn't seem worthwhile resurrecting it because for premultiplied
buffers, if alpha is 0, then so are the color channels
normally.

This patch eliminates the mask_bits and changes the fetchers to just
assume it is 0xffffffff if mask is non-NULL.