[Bf-funboard] Paint brush scale in BU rather than in pixels

[Maxim Therrien]

Hi all, today there was an user on IRC who asked if they could keep their
paint brush size from changing when they zoomed in and out (The brush size
is currently relative to screen pixels).

The user and I thought it would be nice to have such a functionality in
Blender (scaling the brush in BUs instead of surface size), and I was
wondering if it was possible to also show the brush warping along the
geometry when it hits a corner, for example.