Teach Layer and its implementations, RenderObject and its implementations, and PaintingContext to reuse engine layers. The idea is that a concrete RenderObject creates a Layer and holds on to it as long as it needs it (i.e. when it is composited, and the layer type does not change). In return, each Layer object holds on to an EngineLayer and reports it to the engine via addRetained and oldLayer. This allows the Web engine to reuse DOM elements across frames. Without it, each frame drops all previously rendered HTML and regenerates it from scratch.

This relands flutter/flutter#31701 with missing const added

This reverts commit 549b4126.

## Description

These will help identify where the matrix starts to get wrong. 

Also fixed `RenderFittexBox` to no longer paint with empty child which previously triggered invalid matrix computations (NaN with dividing by 0). See also https://github.com/flutter/flutter/pull/7489

## Related Issues

https://github.com/flutter/flutter/issues/31650
https://github.com/flutter/flutter/issues/31700
https://github.com/flutter/flutter/issues/7431

## Tests

* RenderFittedBox does not paint with empty sizes

* Composite physical layers even if elevation is 0

This reverts commit 803b15e0.

Reland composite physical layers and update tests

This reverts commit 58fb1832.

* Always composite PhysicalModels
* Update tests to check layer tree instead of canvas for Physical shadows

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Revert "Revert "Enable taking screenshots of arbitrary RenderObjects from a running a… (#20637)" (#21395)" (#21448)

This reverts commit 5b5a5b82.

This reverts commit 3306fc10.

Enable taking screenshots of arbitrary RenderObjects from a running application from within the inspector.

Key functionality is in the added _ScreenshotPaintingContext class.

* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* Use RenderAnimatedOpacity within AnimatedOpacity widget (#15466)

* Fixed minor bug in RenderAnimatedOpacity

* Updated protected API for ImplicitlyAnimatedWidget

This adds a toImage function to RenderRepaintBoundary that returns an uncompressed raw image of the RenderRepaintBoundary and its children. A device pixel ratio different from the physical ratio may be specified for the captured image. A value of 1.0 will give an image in logical pixels.

For backward compatibility we keep supporting specifying the shape as a
combination of MaterialType and borderRadius, and we just use that as a
default when shapeBorder is null.

To cleanup the implementation if shapeBorder was not specified we just
translate the specified shape to a shapeBorder internally.
I benchmarked paint, layout and hit testing, with the specialized shape
clippers vs. the equivalent path clippers and did not see any
significant performance difference.

For testing, I extended the clippers/physicalShape matchers to match either the
specialized shape or the equivalent shape.

This CL also refactors common logic for RenderPhysicalModel and
RenderPhysicalShape into a base class _RenderPhysicalModelBase.

* Oct 12 10:12am

* implicit_semantics_test.dart passes

* refactoring

* works in nice

* minor rename

* more doc comments

* to be explicit check better

* fix test

* ++

* ++

* semantics_9_test (BlockSemantics) and implicit_semantics_test are passing

* doc updates

* tiny refactor

* fix static errors in tests

* fix gesture detector

* ++

* ++

* geometry

* ++

* remove noGeometry

* revert test

* +

* all tests but scrolling/clipping pass

* clipping works

* scrolling halfway

* sliver tests pass

* ALL TESTS PASS

* SemanticsNode changed

* docs and tiny fixes

* card test

* more doc comments

* remove missed print

* more tests

* make test pass on Linux

* remove changes to intellij proj file

* review comments

* refactor to ensureAction + some related doc fixes

* Update docs for markNeedsSemanticsUpdate

* rewording

* rewording

* ensureAction test

* ensureAction test

* ensureAction test

* more tests

* refactor to allways reset node

* tiny fixes

* more test

* doc fixes

* one more test

* review comments

On Fuchsia, the system compositor is responsible for drawing shadows
for physical model layers with non-zero elevation.

Also fixed a bug where _needsCompositing was not being cleared prior
to updating it.

If the size is empty, then _updatePaintData will produce an invalid transform
that yields a NaN canvas bounds rectangle

Fixes https://github.com/flutter/flutter/issues/7431