New compositor pipeline for paintbrushes

I hadn't originally wanted to do this, given the complexity of PD's
central compositor, but there's no escaping the performance benefits.
The painting pipeline is just different enough to require some specific
tweaks, particularly when it comes to compositing order (as we have to
composite the paint stroke onto the target layer out-of-order), so we're
gonna get better long-term performance with a dedicated paint
compositor.

At present, the brush stroke compositor uses a very similar strategy to
the regular compositor.  Specifically, it re-renders the entire brush
area on every screen refresh.  An obvious future optimization would be
rendering only the regions of the brush layer that have changed, and
when I have time, you can bet I'll tackle that!  (The calculations for
this are going to be a bit complicated, as we don't currently track any
of this information... but the responsiveness benefits will likely be
worth the trouble.)

That said, performance of the current implementation is still
surprisingly competitive with GIMP and Paint.NET on large multilayer
images.

Besides performance benefits, the new compositor pipeline correctly
implements the behavior of painting on a layer with a non-standard
opacity, blend mode, or alpha inheritance.  The paint stroke and target
layer are precomposited *first*, and the result is then blended using
the current layer's settings.  This gets complicated quickly as the
paint brush can have its own opacity, blend mode, and alpha settings,
but I believe everything is sorted.

Unfortunately, PD's "merge layers" tool - which performs the final paint
stroke merge when the mouse is released - does not behave identically.
I'll be updating it shortly to support a "paint" mode, which has some
different considerations from "normal" mode.