Add support for unorm/snorm (for fp32 and fp16)

[xoofx]

[CodeSmile-0000011110110111]

+1

Just encountered odd flickering with normals set to VertexAttributeFormat.SNorm16
I suspect this is due to half4 representing Float16 rather than SNorm16.

I calculate normals with float4 and create new half4() from that on assignment. I am getting odd changes in lighting, especially when planes are axis-aligned. It gets worse if I calculate normals all the way with half4.