VTK should render transparent materials with glTF

[Vinggui]

Hi Drake team,

I have been working on some models in glTF and VTK that contain transparent walls. They can be successfully loaded into Drake, but when rendering with an RGB sensor, they do not show any transparency. Interestingly, the Wavefront model version can have transparency assigned.

Note: Meshcat does render the transparency.

Describe the solution you'd like
Models in glTF can represent transparency with alpha values + "alphaMode".

Describe alternatives you've considered
Maybe glTF can use a more sophisticated KHR extension, allowing better "thin" walls to be rendered

cubes_example.zip