Fixed/Improved float3.Normalize()

float3.Normalize returns the given vector if its length is zero (aka
null vector)
Optimized float4x4.LookAt()

Using a method provided in the DirectX documentation. Probably a bit
faster than what we had.
Meh. JSIL doesn't like IsPositiveInfinity.

Replaced it with a Zero check. We should replace that with some epsilon
soon.