[Impeller] add mechanism for sharing bdf inputs. (#55701)

Introduces a mechanism to allow backdrop filters to 1) share backdrop inputs and 2) fuse filter applications for faster blurs.

This is a proposed solution to https://github.com/flutter/flutter/issues/131568

Implemented:
* Developer can specify a "backdrop id" which indicates that a backdrop layer should share the input texture and potentially cached filter for a layer.
* Removes second save layer for each backdrop filter
* Removes save layer trace event for backdrop filter
* Can fuse backdrop filters if there is more than one identical filter

TBD:
* Adjust heruristic to avoid applying bdf filter to entire screen

Suggestions: applying a bdf should be a distinct operation from a save layer in the DL builder/dispatcher. The saveLayer implmenentation in the impeller dispatcher is super convoluted because it needs to handle both.

### Video

Video starts with normal bdf then I hot reload to specify that the bdfs share inputs/filters. This is running on a pixel 8 pro

Change to the macrobenchmark app is just:
```dart
  Widget build(BuildContext context) {
    Widget addBlur(Widget child, bool shouldBlur) {
      if (shouldBlur) {
        return ClipRect(
          child: BackdropFilter(
            filter: ImageFilter.blur(sigmaX: 5, sigmaY: 5),
            backdropId: 1, // Added ID
            child: child,
          ),
        );
      } else {
        return child;
      }
    }
```

https://github.com/user-attachments/assets/22707f97-5825-43f1-91b4-1a02a43437f5

Requires framework changes in https://github.com/jonahwilliams/flutter/pull/new/backdrop_id