Support placeholder attribute for ZoomableAsyncImage

[minas1]

Coil's AsyncImage has a placeholder attribute: placeholder: Painter? = null.

Please add support for it in ZoomableAsyncImage.

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In the docs there's mention of using placeholders using placeholderMemoryCacheKey() but in Coil's website this is used for regular views, not Compose. And even if it can be used with Compose it would be much easier to support the placeholder attribute directly.

[saket]

Good point, let's add parameters for placeholder, error and fallback. Wanna send a PR?

[MV-GH]

You can get all these already by setting them on the imageLoader and the ImageRequest

[saket]

Yea you can indeed set placeholders through ImageRequest.Builder but it only accepts drawables and resources. You can't use painters through telephoto yet.

@minas1 fwiw placeholderMemoryCacheKey is different from placeholder. It is useful for creating shared element transitions by using a previously loaded image in lower quality as the placeholder for loading a larger resolution variant.