fix(image): fix pixelDensity idempotence and high-DPI canvas resize

[Pcmhacker-piro]

Resolves #9152

Changes:

• In src/image/p5.Image.js, fixed pixelDensity(density) to derive logical dimensions from physical canvas dimensions (this.width = this.canvas.width / density, this.height = this.canvas.height / density). This makes repeated calls idempotent (no compounding dimension shrinkage) and allows resetting pixelDensity(1) to accurately restore original dimensions.
• In src/image/p5.Image.js, fixed resize(width, height) to scale the backing canvas and temporary canvas by this._pixelDensity (width * pd, height * pd). This prevents high-DPI images from having their canvas backing store downscaled to logical size, which caused subsequent get() calls to return [0, 0, 0, 0] as out-of-bounds and set() to fail writes.
• Fixed Color reference in Image.prototype.set().
• Added unit tests in test/unit/image/p5.Image.js covering:
  • pixelDensity() getter and setter behavior
  • Repeated pixelDensity() idempotence and restoring dimensions
  • Non-positive density fallback to 1
  • Backing canvas scaling with pixelDensity > 1 in resize()
  • Full coordinate access with get() and set() after resizing a high-DPI image

PR Checklist

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

AI Usage Disclosure

AI tools were used assistively to help write unit test cases and verify edge cases. The changes have been tested and reviewed locally.

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Thank You!

[Pcmhacker-piro]

Hi @ksen0, could you please take a look at this PR when you have a moment? It resolves #9152 by ensuring p5.Image.prototype.pixelDensity idempotence and properly scaling the backing canvas during resize(). Comprehensive unit tests have been added, and both tests and linter are passing. Thank you!