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]

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