[Bf-codereview] On-surface brush (issue 5554059)

[NicholasBishop at gmail.com]

Sergey noted on IRC that the on-surface brush can look bad with the
layer brush. This is due to the height of the layer brush not being
limited to the brush size. We could "fix" by adding such a limit, but
not sure if in other cases a very tall layer brush is useful.

The grab brush has a similar issue, once the grab "spike" becomes
sufficiently long to exit the brush sphere, the outer border of the
sphere gets drawn over the spike.

http://codereview.appspot.com/5554059/