gaussian density gpu calculation fixes

src/mol-geo/representation/structure/units-visual.ts

...epresentation/structure/visual/gaussian-density-volume.ts

src/mol-gl/renderable/direct-volume.ts

src/mol-gl/shader/direct-volume.frag

src/mol-gl/shader/gaussian-density.frag

src/mol-gl/shader/gaussian-density.vert

src/mol-gl/webgl/texture.ts

src/mol-math/geometry/gaussian-density/gpu.ts