improved brush properties

[bloerwald]

• random pressure
• auto-lowered pressure when used at textures border(eg roads)
• maybe some figure brush(cracks?)

completely customizable brushes have also come up in the past.

[zarkeven]

example of how they have come up: https://bitbucket.org/berndloerwald/noggit6/src/6.x/misc/lua_brushes/