Make eraser a bit larger than brush

[normanrz]

When trying to erase something you just brushed, it is helpful to have an eraser-brush that is slightly larger than the draw-brush. Otherwise, it is quite finicky to remove all the voxels.
Not sure, if the eraser should always be slightly larger than the brush (e.g. 5% larger) or just by default.

This is inspired by the UI of microscopy image browser (MIB).