🚚 Move uForOpening to standard uniforms

docs/how-to-create-new-effects.md

@@ -234,6 +234,7 @@ if (utils.isInShellProcess()) {
       const declarations = `
         // The code below injects some standard uniforms which will be updated during the
         // animation. This includes:
+        // bool      uForOpening: True if a window-open animation is ongoing, false otherwise.
         // sampler2D uTexture:    Contains the texture of the window.
         // float     uProgress:   A value which transitions from 0 to 1 during the entire animation.
         // float     uTime:       A steadily increasing value in seconds.