#390 Add RoundedRectanglePolygon and PathBuilder helpers.

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Description

Fixes #9

This pull request introduces support for rounded rectangles in the drawing library by adding a new RoundedRectanglePolygon shape and integrating it with the PathBuilder API. It also includes comprehensive tests and a reference image to ensure correctness and visual fidelity.

New Shape and API Enhancements:

• Added the RoundedRectanglePolygon class, which defines a rounded rectangle shape with configurable corner radii and provides transformation support.
• Extended the PathBuilder class with multiple AddRoundedRectangle overloads to easily add rounded rectangles to paths using various parameter types (rectangle, radii, coordinates).

Testing and Validation:

• Added new tests in RoundedRectangleTests.cs to verify the correctness of the RoundedRectanglePolygon implementation, including bounds, flattening, corner shapes, scaling, and transformation behavior.
• Added tests in PathBuilderTests.cs to ensure that the new AddRoundedRectangle methods produce paths equivalent to direct construction of RoundedRectanglePolygon.
• Added a test in DrawingCanvasTests.Primitives.cs to compare rendering output of filled and stroked rounded rectangles against a reference image.
• Included a new reference PNG image for visual regression testing of rounded rectangles.

Documentation and Minor Improvements:

• Improved the summary comment for RectangularPolygon for clarity.