[PATCH] Make entity selection faster

[Josh]

Right now, on every mouse movement event we call EntitySelection::PickEntitiesAtPoint. This sometimes happens multiple times per frame. The existing function loops through every entity that is selectable and computes their distance from the mouse which is very inefficient.

Here's a ~hour long profile of a normal game in 0AD: Image https://googledrive.com/host/0BwIjnJSd7iThRXRiN0ExcVVaeDQ/cpuprofile5.svg As you can see, entity selection is a good 1.5%. (I've seen over 3% in some profiles) The in-game profiler also confirmed the problem showing ~5-15 calls of the function per frame, each taking ~50ms.

The attached patch improves the situation by getting a smaller group of entities from the range manager's spatial subdivisions before checking the distance of each one. Initial measurements from the in-game profiler show the new function taking ~45us which is an improvement of over a hundred times.