Sculpt-dev: fix memory corruption

Sculpt-dev: Fix color filter not auto
creating a color layer.

* This is probably something I broke.
There was a SCULPT_has_colors() call that
wasn't needed.

Sculpt-dev: fix memory corruption

Sculpt-dev: fix broken color filter, alpha
was being set to garbage value.

Sculpt-dev: fix thread contention
in weighted smooth
* Cached face areas are now updated
in a double buffered fashion;
all threads read from one side of
the buffer while the other is written
to by the threads that own a given
face; the buffers are swapped on
each iteration of a tool that uses
face areas.
* Fixes smooth flickering.

Sculpt-dev: support area weighted smooth for PBVH_FACES

* This was actually kind of annoying; the
vertex->face-area-list code is in pbvh and
relies on edge ordering
around verts, but that order is non-trivial for
PBVH_FACES (relying as it does on a vertex->poly
map). This ordering was calculated entirely in
editors/sculpt_paint/sculpt.c, not callable from
pbvh.
* The solution was to add a helper function to pbvh
for building vertex->edge lists from vertex->poly
maps. This is then used by both sculpt.c and the
vertex->face-area-list code.

* Also improved boundary bevel smooth a bit. I'm
thinking of extracting it from SCULPT_neighbor_coords_average_interior
into its own function and possibly its own brush.

Sculpt-dev: fix corruption in dyntopo
fast pbvh leaf builder callback.

Sculpt-dev: don't show input mapping
settings in sculpt context menu

Sculpt-dev: Add brush_eval field to Paint

* Paint now has a brush_eval field which
is used in leu of ->brush if non-null.
* This lets us get rid of all the annoying:
`brush = ss->cache ? ss->cache->brush : BKE_paint_brush`
code. Now it's just BKE_paint_brush.
* Used by SCULPT_run_command.
* Also fixed nasty scene spacing bug.

Sculpt-dev: fix multires crash in last commit