optimize symbol-sort-key implementation

[xabbu42]

This is a follow up from the discussion in #985. The current implementation for symbol-sort-key needs a separate draw call for each value of symbol-sort-key (that is at least my current understanding). But two symbols with different symbol-sort-key value only need to be drawn in separate draw calls if they actually overlap. So it is worth an experiment to use the placement code to determine overlapping symbols and then reduce the number of draw calls to the minimum necessary for the resulting partial order.