Fix Python Support For Image Loaders

[kwokcb]

Changes

• Patches to make image loaders declared in Python to work properly.
  • Fixes image data passing (void*) cannot resolve properly.
  • Virtualize extension support query and pass by reference to get data returned properly.
  • Add in trampoline classes so that derived classes get called properly
  • Fix missing type enumerations in pybind bindings.

Tests

• Sample OIIO Loader test added along with sample image from OpenEXR tests
  • Registers the custom load with ImageHander interface
  • Calls aquireImage which will check extensions EXR is not by default supported so will find the sample OIIO loader
  • Calls virtual loadImage override. Also displays the image using matplotlib if available
  • The MaterialX Image is cached in memory after load.
  • Calls saveImage on the ImageHandler passing the MaterialX Image which will again check extensions and call back to the OIIO loader.
  • Calls saveImage override. Also displays the imsage using matplotlib if available.

Results:

• EXR Image loaded

• EXR Image saved