Made clear behavior in RenderTargetBitmap.CreateDrawingContext optional

[PMahern]

The original behavior is still preserved so this shouldn't be a breaking change for anyone relying on the clear behavior.

What does the pull request do?

The current behavior in RenderTargetBitmap.CreateDrawingContext clears the image every time a DrawingContext is created. This is actually a breaking change from old behavior, I may have missed it but I didn't see it documented as such anywhere. I simply added another CreateDrawingContext(bool clear) function that clears the bitmap if true is passed in and then call it from CreateDrawingContext() with true passed in.

What is the updated/expected behavior with this PR?

Create a RenderTargetBitmap bound to an Image.
Create multiple DrawingContexts via CreateDrawingContext(false) that each draw in different areas of the image.
Both drawn shapes should remain in the image.

Checklist

• Added XML documentation to any related classes? (I added some documentation to the CreateDawingContext and CreateEditDrawingContext functions about the difference in behavior)

Fixed issues

Fixes #12792

[PMahern]

@dotnet-policy-service agree

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0039249-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0039255-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[PMahern]

Here's some pictures of the difference in behavior. Running the RenderDemo on the RenderTargetBitmap page (which renders a rotating square) shows this after a few seconds using CreateDrawingContext() (this is with no other calls to clear the image, just creating a DrawingContext and drawing to it).

Changing it to use CreateEditDrawingContext() allows the renders to accumulate in the same image as the square rotates, I added some color changing over time to try and make the difference more obvious. I did notice there's no longer a Clear function on the DrawingContext, was that the motivation behind the original change?

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0039331-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0039333-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0039349-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]