update basic-selection tool to use selection buffer

[strattonbrazil]

The BasicSelect tool used by a lot of cursor tools uses a lot of screen-project calls, which is slow and problematic when certain points fall off the screen. For example, when doing a triangle intersection for prehighlighting, if one vertex is off screen, it will create a very, funky triangle that doesn't intersect as expected. Many selection operations should be able to just read from this buffer.