Improve image loading performance

[dfabulich]

1. We'd configured Coil to load images at their original size, but Coil refuses to use its memory cache when you do that. Now, we're passing it an arbitrary size, and it's magically happy with that.
2. We're now remembering the ImageRequest model, ensuring its stability for recomposition

Before this PR, skiptools/skipapp-showcase#78 was loading each SVG and decoding it as you scroll, on every frame. ☠️

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• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
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• OPTIONAL: I have added an example of any UI changes in the Showcase sample app
  Create a view with 600 copies of the same SVG (stress test) skipapp-showcase#78

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Cursor generated the code, once I figured out that it was never using its memory cache. I tested it with skiptools/skipapp-showcase#78

[marcprux]

Great improvement, thanks!