fix: Dispose TextBoxComponent image cache properly

[spydon]

Description

Since we didn't dispose of the Image object that was used as a cache in TextBoxComponent we had a memory leak.

Side note: I wonder if it is really worth using PictureRecorder in that class.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Breaking Change

• No, this is not a breaking change.

Related Issues

Fixes: #1462

[erickzanardo]

One nit, but LGTM already

[st-pasha]

I think there's still a possibility of a leak here if redraw() is called multiple times in a row. Not sure how to test for this though.

I believe the following version of redraw would be more robust:

  Future<void> redraw() async {
    final newSize = _recomputeSize();
    final image = await _fullRenderAsImage(newSize);
    cache?.dispose();
    cache = image;
    size = newSize;
  }