DRC: some issues with edge layers

[klayoutmatthias]

Here are some findings related to edge layers:

• NOTE: "dots" (single-point edges) in general do not participate in boolean operations or operations reading the edge collection in "merged semantics" mode. A workaround is to switch to "raw" mode for edge layers containing dots. This is a solution for example for the result of an "intersections" operation.
• BUG: "andnot" in deep mode falls back to flat mode.
• BUG: "intersections" copies the first layer instead of giving empty results if the second layer is empty.

[klayoutmatthias]

A fix is provided with this patch: 91e68ce