feat: Refactor screenshot capture for improved performance and reliab…

[marandaneto]

…ility

This commit refactors the captureScreenshot method in ScreenshotCapturer
to improve performance and reliability. The changes include:

• Synchronous Setup: The method now performs synchronous operations first
  (getting context, render object, calculating pixel ratio, and getting the
  initial image) before executing the main process asynchronously. This
  ensures that the initial setup is done on the main UI thread, as required.

• Asynchronous Processing: The main image processing and masking logic
  are now executed asynchronously using a Future and Completer,
  preventing UI jank.

• Snapshot Management: The _views Expando is replaced with a
  SnapshotManager to manage the status of the view, improving code
  organization and maintainability.- feat: Refactor screenshot capture for improved performance and reliability

• chore: process image in completable future example

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

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