direct volume rendering improvements

src/mol-geo/geometry/direct-volume/direct-volume.ts

src/mol-geo/geometry/geometry.ts

src/mol-geo/representation/structure/index.ts

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

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

src/mol-geo/representation/structure/visual/util/common.ts

src/mol-geo/representation/volume/direct-volume.ts

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

src/mol-gl/renderable/schema.ts

src/mol-gl/webgl/render-item.ts

src/mol-gl/webgl/texture.ts

src/mol-gl/webgl/uniform.ts

src/mol-math/geometry/common.ts

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

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

src/mol-model/structure/structure/unit/gaussian-density.ts