[Impeller] Add support for ImageFilter.shader

[jonahwilliams]

Copy-pasta docs:

  /// Creates an image filter from a [FragmentShader].
  ///
  /// The fragment shader provided here has additional requirements to be used
  /// by the engine for filtering. The first uniform value must be a vec2, this
  /// will be set by the engine to the size of the bound texture. There must
  /// also be at least one sampler2D uniform, the first of which will be set by
  /// the engine to contain the filter input.
  ///
  /// For example, the following is a valid fragment shader that can be used
  /// with this API. Note that the uniform names are not required to have any
  /// particular value.
  ///
  /// ```glsl
  /// #include <flutter/runtime_effect.glsl>
  ///
  /// uniform vec2 u_size;
  /// uniform float u_time;
  ///
  /// uniform sampler2D u_texture_input;
  ///
  /// out vec4 frag_color;
  ///
  /// void main() {
  ///   frag_color = texture(u_texture_input, FlutterFragCoord().xy / u_size) * u_time;
  ///
  /// }
  ///
  /// ```
  ///
  /// This API is only supported when using the Impeller rendering engine. On
  /// other backends a [UnsupportedError] will be thrown. This error can be
  /// caught and used for feature detection.

Fixes https://github.com/jonahwilliams/flutter_shaders/issues/34 Fixes https://github.com/jonahwilliams/flutter_shaders/issues/26 Fixes https://github.com/flutter/flutter/issues/132099

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[jonahwilliams]

@flar had feedback that he'd like me to separate the ColorSource type from the Filter type, so I'll follow up with that next.

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[jonahwilliams]

From offline discussion @flar and I agree that this is more of a ColorFilter than an ImageFilter, as we cannot change the render bounds (except for flooding the clip).

To do an ImageFilter, we'd ideally have a means for the developer to specify the bounds transformation based on the input bounds (just a matrix isn't good enough). However, as the developer is writing Dart and the engine is running C++, we'd either need to hack together some sort of serializable DSL or else allow the engine to execute dart code on the raster thread (with some restrictions).

This will be explored in a latter patch.

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[jonahwilliams]

This on hold indefinitely. Its also not really a color filter, and I'm concerned we're going to get stuck with bad semantics so I'm going back to the drawing board.

[jonahwilliams]

Okay, I had more thoughts about this:

• Its not a fully functional ImageFilter, because we can't control the bounds.
• Its not a color filter either, as exposing the input via a texture allows developers to sample from locations other than the current pixel.

As a compromise, I think we should stick with calling it an ImageFilter for now. Its kind of a secret third thing, but that would be difficult to express in the existing API without other intrusive changes that I don't think are worth it. We may find a good way to express the potential bounds transformations in the future.

We could also add a ColorFilter.shader, that would be required to use a more constrainted sampling technique that we could use for further optimizations.

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[jonahwilliams]

From offline discussion, lets add a Matrix option to provide some means of transforming the input pixels.

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[jonahwilliams]

NVM we're not going to do the matrix approach. Adding @gaaclarke for the impeller side of things

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