add sculpting options

[giplt]

Sculpting the elevation map is rather primitive at the moment. A number of relatively simple features could make it a lot more useful:

• Configurable line width so that projected channels or ridges can be broad or narrow on demand.
• Allow for level sculpting in addition to interpolation sculpting: It could be useful to manipulate a map along a fixed elevation.
• Free form sculpting: set a level (by clicking elsewhere in the map?) and start drawing like with a pencil or brush tool. Potential drawback: this could add a lot of data to the settings data encoded in the share link mechanism.