This adds a new property to the sculpt vertex color paint brush to limit
the area of the brush that is going to be used to sample the wet paint
color. This is exactly the same concept as normal radius and area radius
that exist for sculpting brushes for sampling the surface depth and
orientation.

When working near color hard edges, this allows to prevent the color
from the other side of the edge to blend into the wet paint.

With 1.0 (the previous default) wet paint radius, as soon as the brush touches
one vertex of the other color, the wet paint mix color changes, making it
impossible to maintain the border between the two colors.