[Impeller] ImageFilter.shader implicit backdrop sampler cannot be configured with FilterQuality #188365

Use case

When using BackdropFilter with a custom FragmentShader via ImageFilter.shader(), the engine implicitly captures the backdrop and binds it to the shader's first sampler slot.

Currently, there is no Dart API to configure the FilterQuality of this implicitly bound texture. Impeller defaults the sampler to Nearest-Neighbor.

For high-fidelity visual effects (e.g., real-time glass refraction, lens distortion, or liquid effects), nearest-neighbor sampling of the backdrop causes severe stair-step aliasing on high-contrast background edges when the UV coordinates are distorted by the shader.

Proposal

Expose a way to configure the FilterQuality for the implicitly bound texture when using ImageFilter.shader(). This could be done by:

1. Adding a FilterQuality parameter to ImageFilter.shader(...)
2. Or allowing BackdropFilter / BackdropFilterLayer to pass a FilterQuality that applies to its generated texture.

Note: This is an architectural gap analogous to #77165 which notes the exact same missing parameter for ShaderMask.

Workaround

Currently, the only workaround is to manually implement a 4-tap bilinear filter inside the GLSL fragment shader, which incurs a 4x texture sampling performance penalty on every pixel:

// Manual bilinear workaround inside the FragmentProgram
vec2 px = uv * physSize - 0.5;
vec2 p0 = floor(px);
vec4 c0 = texture(tex, (p0 + vec2(0.5, 0.5)) * invPhys);
// ... + 3 more fetches and mix() ...

Having engine-level control over the sampler's FilterQuality would eliminate the need for this expensive workaround.

Filing this new issue ensures it gets triaged by the team currently working on Impeller shaders, rather than getting buried at the bottom of a 2021 Skia thread!

Cheers