Perform BSP polygon splitting and orientation selection in a single step.
This eliminates redundant testing of vertices for orientation with
respect to the splitting polygon. Previously, up to 3 sets of tests
were made (once to determine whether the polygon was split, then once
during the split, and then finally to determine the orientation of the
Merging these steps in order to reuse calculated values also eliminates
the possibility that different calculations in testing and splitting
could be inconsistent.
(cherry picked from commit 3e9082bfc63be6b304543ee499354fd71ea063ae)