to assemble Beziers into outer and inner loops, and find those
loops made up of entities with filled styles. The open paths are
maintained in a separate list, and we assemble as many closed paths
as possible even when open paths exist.
This changes many things. The coplanar check is now performed on
the Beziers, not the resulting polygon. The way that the polygon is
used to determine loop directions is also modified.
Also fix the mouse behavior when dragging a point: drop it when the
mouse is released, even if it is released outside the window, but
don't drop it if the pointer is dragged out of and then back into
our window.
Also special-case SSurface::ClosestPointTo() for planes, for speed.
[git-p4: depot-paths = "//depot/solvespace/": change = 2058]
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