Allow transparent feature symbols to add to each other

Idea created by jnowlin on Aug 7, 2012
    Implemented
    Score520
    One way of showing density is to make all symbols transparent and then allow them to add to eachother (make darker, or make less transparent) as they lay over eachother. Currently when the symbols are set to transparent the whole layer becomes transparent at the same level. I would like the ability to apply a new mode of transparency to a symbol on a point feature which is transparent, yet where it intersects with other symbols becomes darker &/or more opaque; example attached.
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