Walls should have 'overlap' as well as 'mitre' corner option

[brunopostle]

Currently walls are mitred if they meet the 'external' condition. This happens within multi-segment walls and at the ends, the expectation is that 'external' walls wrap around the building even if the style varies, and so can be mitred.

Internal walls ('internal' condition) and walls drawn along other traces are not mitred.

Where 'internal' walls meet external boundaries (i.e. there is an entry in the 'normal' map), the end should be cut back to avoid overlap with the 'external' wall, the angle should be taken from the normal direction (preventing overlap of internal walls at junctions would be nice somehow).

There should be an option to create 'overlap' corner connections for walls in addition to 'mitre', this will cut back the longest segment and extend the shortest segment at each corner. Walls with shallow angle corners should still be mitred. Ends of open walls will still be mitred unless we can implement a system of priority for different wall styles, or simply force shorten/extend on a per-style basis. (reported by @fbpyr)