wip, calculate gaussian density group ids on gpu

.../representation/structure/visual/gaussian-surface-mesh.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/shader/utils/decode-float-rgba.glsl

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