Fix WebGL shader and texture memory leak on sketch removal

[Sanchit2662]

Summary

This PR fixes a critical WebGL lifecycle issue where shader programs and textures were never explicitly released from GPU memory when a sketch was destroyed. As a result, applications that repeatedly create and remove WebGL sketches (e.g. hot-reload editors, instance-mode apps, long-running installations) experienced unbounded GPU memory growth.

The fix introduces explicit dispose() methods for WebGL-backed resources and registers a renderer-level cleanup hook that is automatically invoked when sketch.remove() is called.

---

Impact

Before

• WebGL shaders, programs, and textures remain allocated after remove()
• GPU memory grows unboundedly in multi-sketch or hot-reload scenarios
• No user-level workaround is possible

After

• All WebGL shader and texture resources are explicitly released
• GPU memory remains stable across repeated sketch creation/destruction
• Instance-mode and long-running WebGL sketches behave reliably

---

Changes

1. p5.Shader.dispose()

Adds an explicit teardown path for shader GPU resources.

dispose() {
  if (!this._gl || !this._glProgram) return;

  this._gl.detachShader(this._glProgram, this._vertShader);
  this._gl.detachShader(this._glProgram, this._fragShader);
  this._gl.deleteShader(this._vertShader);
  this._gl.deleteShader(this._fragShader);
  this._gl.deleteProgram(this._glProgram);

  this._glProgram = null;
}

• Properly detaches shaders before deletion
• Deletes both shader objects and the linked program
• Guards against double deletion or uninitialized shaders

---

2. p5.Texture.dispose()

Ensures GPU textures are freed when no longer needed.

dispose() {
  if (this.glTex && !this.isFramebufferTexture) {
    this._renderer.GL.deleteTexture(this.glTex);
    this.glTex = null;
  }
}

• Releases GPU texture memory
• Skips framebuffer-managed textures to avoid double-freeing

---

3. Renderer-level cleanup hook (p5.RendererGL)

Introduces a centralized cleanup method that disposes all WebGL resources owned by the renderer.

Key resources cleaned:

• Default and user shaders
• Cached textures (this.textures)
• Filter shaders and filter layers
• Framebuffers and retained-mode buffers
• Empty texture singleton

The cleanup is automatically invoked on sketch teardown:

this._pInst.registerMethod(
  'remove',
  this._cleanupWebGLResources.bind(this)
);

---

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated
• [Unit tests] are included / updated

[Sanchit2662]

Hi @davepagurek,
This PR fixes a WebGL lifecycle issue where shader programs and textures weren’t being released when a sketch was removed, which could lead to unbounded GPU memory growth in long-running or hot-reload scenarios. The update adds explicit cleanup for WebGL resources and ensures everything is properly disposed on sketch.remove().

Whenever you have time, I’d appreciate a review. Thanks!