OffScreen - ScreenshotAsync incorrectly returned null on first Paint call
OffScreen - ScreenshotAsync incorrectly returned null on first Paint call

Resolve a sequence issue where the Paint event is called before the buffer is updated, the first call
to ScreenshotAsync would return null incorrectly even when the paint event fired.
Add internal AfterPaint event, this event is called regardless of args.Handled
If args.Handled is true and ScreenshotAsync is called then the TaskCompletionSource returns an Exception

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