Make ImageFilter objects comparable and printable.

This will help in areas in the Widget and RenderObject trees which try to avoid marking objects for updates if a setter is called with the same value (previously all ImageFilter objects would compare as not equal and appear to be new values).

Implement basic Picture.toImage via BitmapCanvas and update the dartdocs.

Fixes https://github.com/flutter/flutter/issues/31691

* Add matrix4 param to Linear gradients

Remove the extra `decodedCacheRatioCap` parameter, and the
`_frameBitmaps` member from `Codec`. This means that small looped images
will consume more CPU but prevents us from hitting OOM exceptions based
on trying to render multiple larger images.

Also switch to fDisposalMethod for caching frames.

Previously we looped over every single SkCodec::FrameInfo, tracked its
`fRequiredFrame`, and then saved any frames matching those indeces.
Doing this instead avoids that initialization loop and extra data
structure.

* Dart side resize primitives exposed

* Write the codec side changes

* return un-scaled image if we can not allocate bitmap

* Format _instantiateImageCodec calls to be single lined

Move null check for size to be inner

* Address CR comments and make image resize dimensions container

* Round not trunc, also format

* Add tests, remove ImageResizeDims from api surface

* Make placeholder value public

* Make the api side changes

* Add a feature to resize pixels and also add tests

* Fix grammar and add more info

Dart lints added:
* Avoid optional new
* Avoid optional const
* Prefer single quotes
* Prefer default assignment `=`

This default is already set in the framework, but this also needs to be
set in the binding layer to preserve the same behavior when the
deprecated framework parameter is removed.

Fixes flutter/flutter#31650

Also throw if SkVertices::Builder reports an invalid configuration.

Fixes https://github.com/flutter/flutter/issues/31270

* Revert "fix toString (#8688)"

This reverts commit 9fa73367.

* Revert "Make Rect and RRect use 64 bit doubles, and make them const-able (#8565)"

This reverts commit c1231527.

* Make Rect and RRect 64bit and const-able

This reverts commit 0b36d3e2.

* Improve path metrics tests and docs

* Linking lower layer classes to higher layers in API Docs.

Fix the spelling errors in the dartdocs for the engine.

There are no functionality changes here, just documentation fixes.

* Improve PathMetrics

* Use index instead of dirty

* revert unintended change

* space

* Documentation cleanup

* Typo fix

* add ColorFilter matrix support

Reuses the implementation that was previously done for Scene.toImage
(see https://github.com/flutter/engine/commit/20c805c97312fb69e67984847d031339242a9924)

This introduces a breaking API change:
Picture.toImage is now asynchronous and returns a Future<Image>

Fixes https://github.com/flutter/flutter/issues/23621

The defaultClipBehavior is no longer used anywhere in the Flutter framework and Google3.

* Support user-provided font-fallback.

* Use tonic to pass to native

* Handle font families as a fallback vector in LibTxt

* Fix docs

* Concatentate fontFamily lists in dart before passing to engine

* Fix formatting

* Reworked font family matching to search all managers

* Fix formatting

* Proper toString null checking to keep format consistent

* Formatting

* Move _listEquals out of textstyle as it is not specific to TextStyle and will later be used for paragraphStyle too