fix: spotlight shader code update + example fix (#79)

* fix: sync shader code with drei

* fix: update spotlight example

.storybook/stories/volumetricSpotlight.stories.ts

@@ -169,7 +169,7 @@ function renderDepth() {
  */
 function depthOnResize() {
   renderer.getSize(rendererSize)
-  rendererSize.multiplyScalar(renderer.getPixelRatio())
+  rendererSize.multiplyScalar(renderer.getPixelRatio()) // depth texture will get misaligned if pixel is not multiplied
   volumeMaterial.resolution.copy(rendererSize)
 }