Improvement to codegen of functional graphs in MDL

[niklasharrysson]

This change list improves on codegen of functional graphs in MDL, adding code for default geomprops on the inputs of functions generated from graphs.

For example, if an input in MaterialX has a defaultgeomprop attribute set:

    ...
    <input name="normal" type="vector3" defaultgeomprop="Nworld" uiname="Normal" uifolder="Geometry"
           doc="Input geometric normal" />
    <input name="tangent" type="vector3" defaultgeomprop="Tworld" uiname="Tangent Input" uifolder="Geometry"
           doc="Input geometric tangent" />
    ...

The code generated for this input will include an assignment of the corresponding data as default value in MDL:

    ...
    float3 normal = state::transform_normal(state::coordinate_internal, state::coordinate_world, state::normal()),
    float3 tangent = state::transform_vector(state::coordinate_internal, state::coordinate_world, state::texture_tangent_u(0))
    ...

[jstone-lucasfilm]

This looks good to me, thanks @niklasharrysson!