Add built-in ShaderEffect presets for common sprite effects

[obiot]

Summary

melonJS already has ShaderEffect and GLShader support on any Renderable, but using them requires writing raw GLSL from scratch. Adding a set of built-in, ready-to-use effect presets would dramatically lower the barrier for visual polish.

Proposed Built-in Effects

Hit / Status feedback

• FlashEffect — flash the sprite white (or any color) for hit feedback
• TintPulseEffect — pulse a color overlay (poison green, freeze blue, fire red)
• DesaturateEffect — partial or full grayscale (disabled state, death, petrify)

Visual emphasis

• OutlineEffect — solid color outline / glow around the sprite (selection, hover, collectibles)
• DropShadowEffect — offset shadow beneath the sprite

Transitions

• DissolveEffect — noise-based dissolve (death, spawn, teleport)
• PixelateEffect — progressive pixelation (teleport, transition, retro effect)

Stylistic

• ChromaticAberrationEffect — RGB channel offset (impact, glitch, damage)
• ScanlineEffect — horizontal scanlines overlay

API Sketch

import { FlashEffect, OutlineEffect } from "melonjs";

// one-liner usage
mySprite.shader = new FlashEffect(renderer);

// with options
mySprite.shader = new OutlineEffect(renderer, {
    color: "#ff0",
    width: 2.0
});

// animate via uniforms
const flash = new FlashEffect(renderer, { intensity: 0.0 });
mySprite.shader = flash;
flash.uniforms.intensity = 1.0; // trigger flash, tween back to 0

Implementation Notes

• Each preset is a subclass of ShaderEffect with a sensible default fragment shader and exposed uniforms
• All effects should be parameterizable (color, intensity, speed, etc.)
• Effects that need the sprite's texture dimensions (outline, pixelate) can read from existing uniforms
• Canvas renderer fallback: effects are silently ignored (existing ShaderEffect behavior)
• Effects should be lightweight — a developer should be able to apply/remove them at any time without allocation overhead
• Consider pooling for frequently toggled effects (e.g., flash on hit)

Priority

Start with the 3 most universally used: FlashEffect, OutlineEffect, DesaturateEffect. These alone would cover the majority of use cases.