wip, gpu gaussian density computation

src/apps/canvas/component/representation.tsx

...representation/structure/visual/gaussian-density-point.ts

.../representation/structure/visual/gaussian-surface-mesh.ts

...esentation/structure/visual/gaussian-surface-wireframe.ts

src/mol-gl/renderable/gaussian-density.ts

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

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

src/mol-gl/util.ts

src/mol-gl/webgl/context.ts

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

src/mol-gl/webgl/texture.ts

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

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