MaskEffect: add image/texture-based mask support

[obiot]

Summary

Extend MaskEffect to accept an image or sprite as the mask shape, in addition to geometric shapes (Ellipse, Polygon). This would allow custom transition masks using textures — star wipes, heart reveals, game logo transitions, etc.

Current State

MaskEffect currently accepts:

• Shape presets: "iris" (Ellipse), "diamond" (Polygon)
• Custom geometric shapes: Ellipse, Polygon

The renderer's setMask() only supports geometric shapes, so image-based masks need a different rendering approach.

API Sketch

// image-based mask transition
state.transition("transition", "#000", 500, loader.getImage("star_silhouette"));

// or directly on the camera
camera.addCameraEffect(new MaskEffect(camera, {
    shape: loader.getImage("star_mask"),
    color: "#000",
    duration: 500,
    direction: "hide",
}));

Implementation Notes

The image's alpha channel defines the visible area. Requires different code paths per renderer:

Canvas:

• Fill screen with transition color
• Draw the scaled mask image using globalCompositeOperation = "destination-out" to punch holes where the image has alpha

WebGL:

• Option A: render the mask image with alpha testing into the stencil buffer, then fill where stencil is not set
• Option B: use a custom shader that samples the mask texture and discards fragments based on alpha

Related

• Part of Refactor camera effects into an extensible effect list with scene transitions #1369 (camera effects system)
• MaskEffect: src/camera/effects/mask_effect.ts
• setMask(): src/video/canvas/canvas_renderer.js, src/video/webgl/webgl_renderer.js