Use this delegate to generate custom geometry in the content region of the VisualElement.
You can use the MeshGenerationContext.painter2D object to generate visual content,
or use the MeshGenerationContext.Allocate method to manually allocate a mesh and then fill in the vertices and indices.
This delegate is called during the initial creation of the VisualElement and whenever a repaint is needed.
This delegate isn't called on every frame refresh. To force a repaint, call VisualElement.MarkDirtyRepaint.
Note: When you execute code in a handler to this delegate, don't update any property of the VisualElement, as this can
alter the generated content and cause unwanted side effects, such as lagging or missed updates. To avoid this, treat the VisualElement
as read-only within the delegate.
Additional resources: MeshGenerationContext