Cleanest way here is not do bounding box collision for editmeshes at
all. Decision is taken because:
* Usually we want to do the snapping to the edited mesh anyway (when we
don't the mesh is skipped completely, so we don't need to worry for
extra checks)
* Bounding box is calculated from derived mesh. This means that for
subsurfed meshes for instance, the bounding box may be significantly
smaller than the size of the edit mesh.
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