Look first assignment ordering

[kwokcb]

Look vs material first assignments

This needs a re-work to be look first vs material first.
Just leaving this as a WIP branch for others if interested.

Refined test:

<?xml version="1.0"?>
<materialx version="1.38">

  <standard_surface name="SR_red" type="surfaceshader">
    <input name="base_color" type="color3" value="1, 0, 0" />
  </standard_surface>
  <surfacematerial name="Red" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="SR_red" />
  </surfacematerial>

  <standard_surface name="SR_blue" type="surfaceshader">
    <input name="base_color" type="color3" value="0, 0, 1" />
  </standard_surface>
  <surfacematerial name="Blue" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="SR_blue" />
  </surfacematerial>

  <look name="Look">
    <materialassign name="Red_P" geom="/Preview_Mesh" material="Red" />
    <materialassign name="Blue_P" geom="/Preview_Mesh" material="Blue" />
    <materialassign name="Blue_C" geom="/Calibration_Mesh" material="Blue" />
    <materialassign name="Red_C" geom="/Calibration_Mesh" material="Red" />
  </look>

</materialx>

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