fix(sliceview/base): correct source bounds intersection computation
fix(sliceview/base): correct source bounds intersection computation

Correct bug in computation of fullyVisibleSources that led to sources
that were not fully visible being added to the list.

google/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,funkey/neuroglancer,janelia-flyem/neuroglancer,funkey/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,funkey/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,funkey/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer