Implements #233 cross support region for MLS

Colour MLS can optionally use a cross support region. It is currently a lot slower. In future it should also be adapted to be aware of significant non-crossable edges rather than just using colour information.

added 4 commits

• 3db5760d...39d6bef7 - 3 commits from branch master
• 2d2be65d - Merge with master

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merged

mentioned in commit 1bc79abe