fix: Do not redraw on iOS if background image is 'none'

[sebj54]

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What is the current behavior?

On iOS, views with background-image: none are redrawn multiple times while they shouldn't. Even if you just set a background-color, background-image is automatically set to none (I don't know what is setting this value though).

What is the new behavior?

Views are redrawn only if the view has a background-image, a non-uniform border or a border radius.

Additional information

I identified this problem because my app crashed because of a constantly redrawn background. This PR does not fix the problem in my app (I made other app-specific changes) but I thought that the change is still needed for better performance.

I also could not run the tests (I think the README is outdated because the tests folder does not exist).