refactored intMod/intDiv shader code

src/mol-gl/shader/chunks/apply-marker-color.glsl

src/mol-gl/shader/chunks/common-frag-params.glsl

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

src/mol-gl/shader/chunks/common.glsl

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

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

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

src/mol-gl/shader/utils/read-from-texture.glsl

src/mol-gl/shader/utils/texture3d-from-2d-linear.glsl

src/mol-gl/shader/utils/texture3d-from-2d-nearest.glsl