vtkOpenGLGPUVolumeRayCastMapper: support textures larger than 1024
This merge request backports some changes from https://github.com/Slicer/VTK to upstream, specifically: https://github.com/Slicer/VTK/commit/61cdd20:
When dealing with large opacity or color transfer functions, shrinking the size of the function might add some artifacts. This can be an issue with labelmaps where each entry has a color very different from its neighbors.
This merge request supersedes !482 (closed). As suggested there, a test is added to verify the functionality of vtkOpenGLGPUVolumeRayCastMapper when using large, fine-grained, lookup tables.
This is marked as WIP and no baseline has been provided for the test because vtkOpenGLGPUVolumeRayCastMapper isn't giving the expected output for the test. Swapping out vtkOpenGLGPUVolumeRayCastMapper with vtkFixedPointVolumeRayCastMapper does give the expected output; that suggests the test is valid.