The FixedMul() C implementation would produce off by one results,
causing constant desyncs on 64 bit builds and builds without an
ASM implementation of the function.
This is fixed by shifting instead of dividing, possibly avoiding
rounding errors.
These functions were already here before, and I /swear/ the slope
physics became slightly less glitchy after switching to them...
Only issue is the slope plane mapping code hasn't been properly
converted yet, so they don't render properly for now.
