Add precision control of computations. Fixed, half, and float.

Hi there,

As the title says, would be a good idea to add more control over the type of variables used by each node, or some method to control it. It would be a good way to optimize the shaders and not have to touch the code (very important for mobile developers). Like "color" variables, that actually SF uses float3 or float4 and its useless.

Thank you for considering my proposal!