[vector_graphics] Fix memory leaks and activate leak testing [prod-leak-fix]

[ValentinVignal]

The package manipulates disposable objects. This PR

• Fixes some memory leaks.
• Activates leak testing to make sure disposable objects are correctly disposed.

See the documentation: https://github.com/dart-lang/leak_tracker/blob/main/doc%2Fleak_tracking%2FDETECT.md

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[ValentinVignal]

cc @polina-c

[jonahwilliams]

I don't really understand what this is doing?

[ValentinVignal]

For the test Can render VG with image from test/vector_graphics_test.dart, leak_tracker_flutter_testing was not happy because the ImageInfo was not disposed from this callback was never disposed.

So here I am disposing it. However, disposing it also disposes it image.image, that's why I'm cloning it first.

  Expected: leak free
    Actual: <Instance of 'Leaks'>
     Which: contains leaks:
            # The text is generated by leak_tracker.
            # For leak troubleshooting tips open:
            # https://github.com/dart-lang/leak_tracker/blob/main/doc/leak_tracking/TROUBLESHOOT.md
            notDisposed:
              total: 1
              objects:
                ImageInfo:
                  test: Can render VG with image
                  identityHashCode: 137082098
                  context:
                    start: >
                      #6_______dispatchFlutterEventToLeakTracker_(package:leak_tracker_flutter_testing/src/testing.dart:74:23)
                      #7______FlutterMemoryAllocations.dispatchObjectEvent_(package:flutter/src/foundation/memory_allocations.dart:249:23)
                      #8______FlutterMemoryAllocations.dispatchObjectCreated_(package:flutter/src/foundation/memory_allocations.dart:283:5)
                      #9______new_ImageInfo_(package:flutter/src/painting/image_stream.dart:51:34)
                      #10_____ImageInfo.clone_(package:flutter/src/painting/image_stream.dart:72:12)
                      #11_____ImageStreamCompleter.setImage_(package:flutter/src/painting/image_stream.dart:755:32)
                      #12_____StackZoneSpecification._registerUnaryCallback.<anonymous_closure>.<anonymous_closure>_(package:stack_trace/src/stack_zone_specification.dart:127:36)
                      #13_____StackZoneSpecification._run_(package:stack_trace/src/stack_zone_specification.dart:207:15)
                      #14_____StackZoneSpecification._registerUnaryCallback.<anonymous_closure>_(package:stack_trace/src/stack_zone_specification.dart:127:24)
                      #15______rootRunUnary_(dart:async/zone.dart:1422:47)
                      #16______CustomZone.runUnary_(dart:async/zone.dart:1324:19)
                      #17_____Future._propagateToListeners.handleValueCallback_(dart:async/future_impl.dart:902:45)
                      #18_____Future._propagateToListeners_(dart:async/future_impl.dart:931:13)
                      #19_____Future._completeWithValue_(dart:async/future_impl.dart:707:5)
                      <asynchronous_suspension>
            
            
  
  package:matcher                                              expect
  package:leak_tracker_testing/src/leak_testing.dart 69:24     LeakTesting.collectedLeaksReporter.<fn>
  package:leak_tracker_flutter_testing/src/testing.dart 70:37  maybeTearDownLeakTrackingForAll
  ===== asynchronous gap ===========================
  dart:async                                                   _CustomZone.registerBinaryCallback
  package:flutter_test/src/test_compat.dart 287:3              _tearDownForTestFile
  

Is there a better way you have in mind?