Called whenever the panel should be drawn.
This hook will not run if the panel is completely off the screen, and will still run if any parts of the panel are still on screen.
You can create panels with a customized appearance by overriding their Paint()
function, which will prevent the default appearance from being drawn.
See also PANEL:PaintOver.
Creates a DPanel and overrides its Paint() function to draw a 100x100 pixel black rounded box in the center of the screen.