fix: reduce lag when navigating directories with large image files

[sahinfalcon]

This will fix: #428

Changes so far:

• Skip image processing when preview panel is closed
• Add colour caching to reduce redundant colour space conversions in image
  processing. Cache previously converted colours to avoid repeated RGBA->termenv
  conversions of identical pixels.

So far, this means that:

1. No lag occurs when navigating directories with large images while preview is hidden
2. No lag occurs when performing other operations (e.g renaming files) within directories with large images while preview is hidden
3. Eliminates repeated colour conversions for duplicate pixels

[netlify]

✅ Deploy Preview for superfile canceled.

Name	Link
🔨 Latest commit	a48762d
🔍 Latest deploy log	https://app.netlify.com/sites/superfile/deploys/676ec73abc5f9e0008ed6992

[yorukot]

That great! thanks a lot!