fix: improve robustness of sliceview vertex shader
fix: improve robustness of sliceview vertex shader

This makes the sliceview vertex computation much more robust to viewport
plane normals almost, but not perfectly axis aligned.  Previously, in
such situations, incorrect vertices were generated, resulting in display
artifacts.

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