Add a Paint event to GameCanvas for drawing over sprites

GameCanvas is currently sprite/tile-only: there is no way to draw text or shapes over the game view. Effects like a score readout, health bars, dialogue text, or highlighting the sprite under the mouse require pre-rendered images or overlay controls, which is awkward for anything that changes every frame.

Add a Canvas-style Paint event to GameCanvas. Each frame, after sprites and tile layers are drawn, the handler receives a Graphics object and can draw text, shapes, and pictures on top of the game — a lightweight HUD/annotation layer using the same Graphics API already known from Canvas.