Fix writable mat4/mat3 setters for p5.Matrix to restore Camera.slerp() in ortho mode

[nakednous]

This PR fixes issue #7837.

In p5.js v2, Matrix.mat4 is exposed via a getter that returns an internal Float32Array.
Camera.slerp() (and potentially other code) assigns to uPMatrix.mat4 which throws:

TypeError: setting getter-only property "mat4"

This change adds a set mat4(values) on Matrix that:

• only applies to 4×4 matrices,
• expects an array-like of length 16, and
• copies values into the existing Float32Array instead of reassigning it.
• Adds a symmetric set mat3(values) for 3×3 matrices.

This fixes the intended writable behavior (used by Camera.slerp()), while preserving the internal buffer identity required by WebGL bindings and other code that holds references to the matrix storage.

[welcome]

🎉 Thanks for opening this pull request! For guidance on contributing, check out our contributor guidelines and other resources for contributors!
🤔 Please ensure that your PR links to an issue, which has been approved for work by a maintainer; otherwise, there might already be someone working on it, or still ongoing discussion about implementation. You are welcome to join the discussion in an Issue if you're not sure!
🌸 Once your PR is merged, be sure to add yourself to the list of contributors on the readme page !

Thank You!

[nakednous]

Rebasing this PR onto the latest dev-2.0.
I’ve re-tested the fix locally using the built lib/p5.js, and the ortho Camera.slerp() issue is resolved.

[davepagurek]

Hi @nakednous, sorry for the delay on this! I think this approach works, but it might be for the best to not allow these to be settable for safety? @holomorfo @limzykenneth do you have thoughts?

If we don't want to let those properties be settable, rather than cloning + updating the existing matrix here:

p5.js/src/webgl/p5.Camera.js

Lines 1593 to 1594 in a6e0b46

	this._renderer.states.setValue('uPMatrix', this._renderer.states.uPMatrix.clone());
	this._renderer.states.uPMatrix.mat4 = this.projMatrix.mat4.slice();

...maybe we just set the state directly to a clone of the camera's matrix?