added de/encodeFloatRGBA glsl helper functions

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

+float decodeFloatRGBA(vec4 rgba) {
+  return dot(rgba, vec4(1.0, 1/255.0, 1/65025.0, 1/16581375.0));
+}
+
+#pragma glslify: export(decodeFloatRGBA)
\ No newline at end of file

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

+vec4 encodeFloatRGBA(float v) {
+  vec4 enc = vec4(1.0, 255.0, 65025.0, 16581375.0) * v;
+  enc = frac(enc);
+  enc -= enc.yzww * float4(1.0/255.0,1.0/255.0,1.0/255.0,0.0);
+  return enc;
+}
+
+#pragma glslify: export(encodeFloatRGBA)
\ No newline at end of file