Increased multi-entity selection performance

Hi everyone,

I've managed to fix some very annoying performance issue caused by trying to select units.
Instead of trying to select units while dragging the mouse over the map, it's now waiting until the mouse has been released.
Furthermore, I've optimized EntitySelection::PickEntitiesInRect's performance by not calculating the screen position of all possible units but instead calculate the map-aligned rectangle based on the screen coordinates for exactly one time.
Despite less matrix operations are done and thus the function will likely be executed faster,
there tend to be selection errors if the camera is rather zoomed out.
This pull-request probably won't get merged but I just wanted to mention a way how this really bothering selection lag can be resolved. :)