Don't hide brushes with invalid shape #12
Description
While modifying the shape of a brush, it occasionally happens that the shape becomes "invalid" (read: somewhat undesired by the Chisel code). This is easy to reproduce, for example, when making a concave shape. Chisel then hides the brush from the scene view: It just disappears instantly while I am still dragging the handles. It would be better if the brush remained visible in some way, with its handles intact and continues to allow me the drag the handles so that I can fix the problem. It's perfectly fine if the brush can't be rendered correctly, then just draw some red wireframe instead to indicate that there is a problem.
