Declares a static method of a VisualElementComponentAttribute struct as a painter that contributes to the owner element's VisualElement.generateVisualContent.
The method must be @@static void M(ref T self, MeshGenerationContext mgc). The
source generator hooks it into the owner's generateVisualContent@@ when the component is added
and unhooks it when the component is removed, and requests an initial repaint. Geometry changes
re-run the painter through the engine, and [StyleProperty]
changes repaint via the generated style handler, so no extra callbacks are needed.