Improve stream handling in Viewer and GraphEditor

[kwokcb]

Issue

Fixes #1495 Fixed #1494

• The glTF loader does not load in multiple texcoords so can't use them if they exist. It does for colors.
• Both the Viewer and GraphEditor force all texcoord node lookups to be 0 so no other set can be used.

Fixes

• Update Mesh class to create "fallback" streams if not found. This is similar action to what the Web Viewer does using three.js. Will handle if normal, tangent, bitangent or color or texcoord sets do not exist. Missing texcoord sets map to texcoord set 0. Missing color sets map to position.
• Change Glsl and Msl renderers to create fallbacks if needed.

Tests

glTF Texcoord Test

• MultiUVTest from glTF Sample Repository. Uses existing texcoord set 1 from glTF file.

<?xml version="1.0"?>
<materialx version="1.38">
  <gltf_pbr name="SHD_Material" type="surfaceshader" xpos="13.768116" ypos="-0.672414">
    <input name="base_color" type="color3" nodename="image_basecolor" />
    <input name="metallic" type="float" value="1" />
    <input name="roughness" type="float" value="1" />
    <input name="occlusion" type="float" value="1" />
    <input name="emissive" type="color3" nodename="image_emission" />
  </gltf_pbr>
  <surfacematerial name="MAT_Material" type="material" xpos="17.391304" ypos="0.000000">
    <input name="surfaceshader" type="surfaceshader" nodename="SHD_Material" />
  </surfacematerial>
  <gltf_image name="image_basecolor" type="color3" xpos="10.144928" ypos="-0.896552">
    <input name="file" type="filename" value="uv0.png" colorspace="srgb_texture" />
  </gltf_image>
  <gltf_image name="image_emission" type="color3" xpos="10.144928" ypos="0.422414">
    <input name="file" type="filename" value="uv1.png" colorspace="srgb_texture" />
    <input name="texcoord" type="vector2" nodename="texcoord" />
  </gltf_image>
  <texcoord name="texcoord" type="vector2" xpos="6.463768" ypos="-0.517241">
    <input name="index" type="integer" value="1" />
  </texcoord>
</materialx>

Missing Stream Test

<?xml version="1.0"?>
<materialx version="1.38">
  <texcoord name="texcoord" type="vector2" xpos="8.666667" ypos="0.870690">
    <input name="index" type="integer" value="10" />
  </texcoord>
  <geomcolor name="geomcolor_color3" type="color3" xpos="8.434783" ypos="-0.810345">
    <input name="index" type="integer" value="100" />
  </geomcolor>
  <checkerboard name="checkerboard_color3" type="color3" xpos="10.637681" ypos="1.146552">
    <input name="texcoord" type="vector2" nodename="texcoord" />
    <input name="uvtiling" type="vector2" value="20, 20" />
    <input name="color1" type="color3" value="0.374083, 0.368595, 0.368595" />
  </checkerboard>
  <surface_unlit name="surface_unlit" type="surfaceshader" xpos="13.797101" ypos="-2.258621">
    <input name="emission_color" type="color3" nodename="geomcolor_color3" />
  </surface_unlit>
  <surface_unlit name="surface_unlit2" type="surfaceshader" xpos="14.000000" ypos="0.931035">
    <input name="emission_color" type="color3" nodename="checkerboard_color3" />
  </surface_unlit>
  <tangent name="tangent_vector3" type="vector3" xpos="8.884058" ypos="3.206897">
    <input name="index" type="integer" value="34" />
  </tangent>
  <bitangent name="bitangent_vector3" type="vector3" xpos="8.565217" ypos="5.801724">
    <input name="index" type="integer" value="200" />
  </bitangent>
  <convert name="convert_vector3_surfaceshader" type="surfaceshader" xpos="12.326087" ypos="3.758621">
    <input name="in" type="vector3" nodename="tangent_vector3" />
  </convert>
  <convert name="convert_vector3_surfaceshader2" type="surfaceshader" xpos="12.028986" ypos="6.232759">
    <input name="in" type="vector3" nodename="bitangent_vector3" />
  </convert>
  <surfacematerial name="surfacematerial" type="material" xpos="16.463768" ypos="-1.620690">
    <input name="surfaceshader" type="surfaceshader" nodename="surface_unlit" />
  </surfacematerial>
  <surfacematerial name="surfacematerial2" type="material" xpos="16.434782" ypos="0.439655">
    <input name="surfaceshader" type="surfaceshader" nodename="surface_unlit2" />
  </surfacematerial>
  <surfacematerial name="surfacematerial3" type="material" xpos="16.695652" ypos="3.629310">
    <input name="surfaceshader" type="surfaceshader" nodename="convert_vector3_surfaceshader" />
  </surfacematerial>
  <surfacematerial name="surfacematerial4" type="material" xpos="16.666666" ypos="5.732759">
    <input name="surfaceshader" type="surfaceshader" nodename="convert_vector3_surfaceshader2" />
  </surfacematerial>
</materialx>

Note

• The render test suite already creates fallbacks for any missing streams before-hand (vs on demand here).