vtkHTGVisibleLeavesSize: default masked cell size value to 0.0
Uninitialized values in the CellSize array for masked cells caused range computation to be wrong. Initialize all indirection array values to 0.0, so that the range computed by linearly browsing the array is correct. All of these values will be overwritten excepted those masked, hence keeping a size of 0.
Fixup !10958 (merged)