Fix irradiance generation in MaterialXView (#1647)

This changelist fixes a regression to environment irradiance generation in MaterialXView, caused by a change to image caching logic for GLSL rendering in MaterialX 1.38.8.  This fix restores the ability to render environment maps that are missing their pre-generated irradiance data, with irradiance being generated on the fly via spherical harmonics.

source/MaterialXView/Viewer.cpp

@@ -402,15 +402,15 @@ void Viewer::loadEnvironmentLight()
  }
 
  // Look for an irradiance map using an expected filename convention.
- mx::ImagePtr envIrradianceMap = _imageHandler->getZeroImage();
+ mx::ImagePtr envIrradianceMap;
  if (!_normalizeEnvironment && !_splitDirectLight)
  {
  mx::FilePath envIrradiancePath = _envRadianceFilename.getParentPath() / IRRADIANCE_MAP_FOLDER / _envRadianceFilename.getBaseName();
  envIrradianceMap = _imageHandler->acquireImage(envIrradiancePath);
  }
 
  // If not found, then generate an irradiance map via spherical harmonics.
- if (envIrradianceMap == _imageHandler->getZeroImage())
+ if (!envIrradianceMap || envIrradianceMap->getWidth() == 1)
  {
  if (_generateReferenceIrradiance)
  {