libdeng2|VectorN<>: Added multiplication assignment operators
Added|API: Fixed-point math variants of BSP lookup/query functions

Added fixed-point math variant of GameMap::bspLeafAtPoint() and
updated callers to use the "correct" version as appropriate.

Reimplemented Partition::pointOnSide() with a more general purpose
equation allowing it to be used to test for points incident with
the line.

Minor performance optimization to lightgrid initialization.