message: gltfio ubershader: provide dummy texture for volume thickness.
  Fixes logcat spam seen with IridescentDishWithOlives.
diff: @@ -218,6 +218,8 @@ MaterialInstance* UbershaderLoader::createMaterialInstance(MaterialKey* config,
  // running into a sampler count limitation. TODO: check if these constraints can now be relaxed.
  bool clearCoatNeedsTexture = true;
  + bool volumeThicknessNeedsTexture = false;
  +
  mat3f identity;
  mi->setParameter("baseColorUvMatrix", identity);
  mi->setParameter("metallicRoughnessUvMatrix", identity);
  @@ -248,6 +250,7 @@ MaterialInstance* UbershaderLoader::createMaterialInstance(MaterialKey* config,
  }
  if (config->hasVolume) {
  clearCoatNeedsTexture = false;
  + volumeThicknessNeedsTexture = true;
  mi->setParameter("volumeThicknessUvMatrix", identity);
  mi->setParameter("volumeThicknessIndex",
  getUvIndex(config->transmissionUV, config->hasVolumeThicknessTexture));
  @@ -280,6 +283,9 @@ MaterialInstance* UbershaderLoader::createMaterialInstance(MaterialKey* config,
  mi->setParameter("clearCoatRoughnessMap", mDummyTexture, sampler);
  mi->setParameter("clearCoatNormalMap", mDummyTexture, sampler);
  }
  + if (volumeThicknessNeedsTexture) {
  + mi->setParameter("volumeThicknessMap", mDummyTexture, sampler);
  + }
  if (!config->hasClearCoat) {
  if (config->hasTransmission) {
  mi->setParameter("transmissionMap", mDummyTexture, sampler);
  