Fix memory leak

Support Inheriting App theme for Invisible Widgets, #54 

* Inherit Theme for Invisible Widget capturing,
* Support  Capturing Invisible widgets with delay and retry

pixel ratio param for captureFromWidget method

fix: issue #49

Removal of compilation warning

This PR fixes the following issue:
```
/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/screenshot-1.2.0/lib/screenshot.dart:45:31: Warning: Operand of null-aware operation '??' has type 'Duration' which excludes null.
 - 'Duration' is from 'dart:core'.
    return new Future.delayed(delay ?? Duration(milliseconds: 20), () async {
                              ^
```

migrate to null safety

version 0.3.0

Add method 'captureAsUiImage()'