Fixes #9000

[NalinDalal]

Resolves #9000

set() on p5.Graphics was significantly slower in 2.x vs 1.11.x because
_getRGBA did a full to('srgb') + structuredClone + map() on every
call, even when the color was already in RGB mode and being reused in a
tight loop.

Changes:

1. Cache _getRGBA results
   • Added #rgbaCache Map on Color, keyed by maxes string
   • First call computes and stores; subsequent calls with same maxes
     return a shallow copy of the cached array
   • Cache is invalidated in setRed, setGreen, setBlue, setAlpha,
     and the _color setter
2. RGB-mode fast path
   • When this.mode === RGB, coords are already sRGB 0-1, so skip the
     to('srgb') conversion and use [...this._color.coords, this._color.alpha]
     directly
   • Non-RGB modes still go through colorjs conversion

AI usage disclosure: I used Kilo (an AI coding assistant) to help
implement and review this change. I understand and take responsibility for
every line of code in this PR. See [AI_USAGE_POLICY.md].

PR Checklist

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

[ksen0]

To show that this fixes the original issue, please report results from the linked benchmark test, at minimum. Thanks!

[NalinDalal]

Confirmed the fix in 88e1550 — benchmark (100×100 buffer, 50 iterations, Brave):

• Average: 2.41 ms
• Min: 1.90 ms
• This matches the v1.11.12 baseline of ~1.85 ms, down from the reported 12.79 ms in 2.3.0.

Hope this resolves your query @ksen0