Work in premult alpha for more modes

Add partial specializations which operate on arrays with premultiplied data
directly for the following combinations:

* Source Over (for the DSTALPHA=true variant as well)
* Destination In (both variants)
* Destination Out (both variants)

Using svg:src-over onto an alpha backdrop within the UI is becoming more
common (masks, Show Background toggle, Pass-through layer groups).  It
makes sense to have this code path work more efficiently.

Addresses issue #21 a bit as well: doing this gets a couple of extra fps
for both the scroll_1x_lazy_all_onscreen_masks and the
scroll_1x_lazy_all_onscreen_nobg test cases. To me it's noticeable in the
app too, if still a bit laggy.

Verified we can use the same template for src-over without penalty to the
no-alpha case, at least on the compiler I use.