[dart:ui] Adds a reusable FragmentShader

[zanderso]

This PR adds a new FragmentShader subclass of Shader that does not require allocating new uniform and sampler arrays on every frame. Instead the float and sampler uniforms are backed by native allocations, whose entries can be updated between uses.

Like an ImageShader, the FragmentShader should be disposed. Instead of copy-pasting that code from ImageShader, this PR also moves the dispose() and debugDisposed methods to the Shader class.

After use of this API propagates to the framework, I'll remove the old API.

[zanderso]

@jonahwilliams wdyt?

[jonahwilliams]

From offline discussion: really like that we can dramatically cut down on typed data copying, and love the additional documentation. My only potential concern is that multiple shaders created from the same builder will have the same backing data, which may or may not be surprising. What if the Shader was more like a paint object and had mutable uniforms? Might be more obvious how that behaves - but this is an educated guess since the shader APIs aren't widely used.

[zanderso]

The InkSparkle in the framework will look roughly like this flutter/flutter#110552.

[jonahwilliams]

I will take an indepth pass on this tomorrow, but so far I really like this API

[dnfield]

nit: if you type these in the declaration tonic will just do this for you.

[zanderso]

That's how I had it at first, but the integer parameters were getting the value of the Dart_Handle rather than the value wrapped by the Dart_Handle. I'm going to leave these as is, and try to make a repro of the issue I was seeing.

[dnfield]

Just nits

[jonahwilliams]

Another approach, if the ergonomics of setFloat aren't to your liking. Just create a view of the original object:

class Float32Uniforms {
  FragmentShader? _instance;
  
  operator[]=(int index, double value) {
     assert(disposedStuff);
     _instance._setFloat(index, value);
   }
}

class FragmentShader {
  Float32Uniforms _floatUniforms;
  Float32Uniforms get floatUniforms => _floatUniforms;
  
  void dispose() {
    _floatUniforms._instance = null;
  }
}

[zanderso]

I trust the VM's ability to inline the simple wrapper that's currently in the PR. Adding more indirection would make me start to worry...

[jonahwilliams]

no prob. might be possible to do "for free" someday with views when/if they come out:

view Float32Uniforms on FragmentShader {
  void operator[]=(int index, double value) => this.setFloat(index, value);
}

class FragmentShader {
  Float32Uniforms get floatUniforms => this as Float32Uniforms;
}

[dnfield]

nit: call super.dispose last, since it might rip something out from under you

[dnfield]

Oh, hm, probably should call _dispose last since it deletes the native peer though...

[zanderso]

Right.

[jonahwilliams]

Lgtm