Add function to extract float3x3 from float4x4

[unpacklo]

It doesn't appear that we have a convenience function to extract the upper left float3x3 from a float4x4. Currently you must extract manually:

return new float3x3(m.c0.xyz, m.c1.xyz, m.c2.xyz);

Add this as a convenience function.

[tlaedre]

In the spirit of HLSL, shouldn't there be an explicit cast implemented from float4x4 to float3x3?

[unpacklo]

Yes, I think being able to do both of these would be good:

var f3x3 = new float3x3(f4x4);
var f3x3 = (float3x3)f4x4;