wip, color support for direct volume isosurface

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

src/mol-geo/geometry/picking.ts

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

.../representation/structure/visual/gaussian-surface-mesh.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/shader/chunks/assign-color-varying.glsl

src/mol-gl/shader/chunks/color-vert-params.glsl

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

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

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

src/mol-gl/shader/utils/decode-float-rgb.glsl

src/mol-gl/shader/utils/decode-float-rgba.glsl

src/mol-gl/shader/utils/decode-id-rgb.glsl

src/mol-gl/shader/utils/encode-float-rgb.glsl

src/mol-gl/shader/utils/encode-float-rgba.glsl

src/mol-gl/shader/utils/encode-id-rgb.glsl

src/mol-gl/shader/utils/my-div.glsl