Clarify usage of the ImageFilter API for fragment shaders. (#12522)

_Description of what this PR is changing or adding, and why:_

- Suggest using the ImageFiltered class in addition to the
BackdropFilter class.
- Fix broken link to BackdropFilter page.
- Call out the automatically-filled uniform values when using
ImageFilter.shader.
- Change the example code to not set a size uniform value. As called out
above, this is incorrect and these set values are ignored.

_Issues fixed by this PR (if any):_

_PRs or commits this PR depends on (if any):_

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---------

Co-authored-by: Shams Zakhour <[email protected]>

Author: b-luk

src/content/ui/design/graphics/fragment-shaders.md

@@ -115,21 +115,27 @@ void paint(Canvas canvas, Size size, FragmentShader shader) {
 
 ### ImageFilter API
 
-Fragment shaders can also be used with the [`ImageFilter`][] API. This allows
-using custom fragment shaders with the [`BackdropFilter`][] class to apply
-shaders to already rendered content. [`ImageFilter`][] provides a constructor,
-[`ImageFilter.shader`][], for creating an [`ImageFilter`][] with a custom
-fragment shader.
+Fragment shaders can also be used with the [`ImageFilter`][] API.
+This allows using custom fragment shaders with the
+[`ImageFiltered`][] class or the [`BackdropFilter`][] class
+to apply shaders to already rendered content.
+[`ImageFilter`][] provides a constructor, [`ImageFilter.shader`][],
+for creating an [`ImageFilter`][] with a custom fragment shader.
+
+Fragment shaders that use the `ImageFilter` API receive some
+values automatically from the engine. The `sampler2D` value at index 0
+is set to the filter input image, and the `float` values at indices 0
+and 1 are set to the image's width and height.
+Your shader must specify this constructor to accept these values (for example,
+a `sampler2D` and a `vec2`), but you should not set them from your Dart code.
 
 [`ImageFilter`]: {{site.api}}/flutter/dart-ui/ImageFilter-class.html
-[`BackdropFilter`]: {{site.api}}/flutter/dart-ui/BackdropFilter-class.html
+[`ImageFiltered`]: {{site.api}}/flutter/widgets/ImageFiltered-class.html
+[`BackdropFilter`]: {{site.api}}/flutter/widgets/BackdropFilter-class.html
 [`ImageFilter.shader`]: {{site.api}}/flutter/dart-ui/ImageFilter/ImageFilter.shader.html
 
 ```dart
 Widget build(BuildContext context, FragmentShader shader) {
-  final screenSize = MediaQuery.of(context).size;
-  shader.setFloat(0, screenSize.width);
-  shader.setFloat(1, screenSize.height);
   return ClipRect(
     child: SizedBox(
       width: 300,