Remove block opacity when changing visibility
This MR reverts f064d4bb (which did not fix the bug it was intended to fix) and instead removes a block's opacity entry when changing a block's visibility to false.
Because visibility and opacity are orthogonal in vtkSMTKResourceRepresentation, it was originally possible for a block to have a visibility of 0 and an opacity of 1. The block would therefore not render, but it would still occlude other blocks. This MR tethers (in)visibility and opacity, so invisible objects do not have an opacity entry in the class's BlockOpacities map.