Add ability to change opacity for compose view

[ASalavei]

Usage example:

val viewController = ComposeUIViewController(
    configure = { opaque = false }, // Make compose scene background transparent
    content = ::SomeComposeNode
)

Related issue: JetBrains/compose-multiplatform#4108

[MatkovIvan]

The code LGTM in general, but

1. We need to agree about public API (transparency vs opaque) to keep it harmonized between platforms. cc @igordmn
2. I'd like to wait a review from @elijah-semyonov since it affects render pipeline/performance

[ASalavei]

1. We need to agree about public API (transparency vs opaque) to keep it harmonized between platforms. cc @igordmn

I named it opaque because it is most commonly used term in iOS (compared to transparency)

[elijah-semyonov]

We discussed it in a call, LGTM

[chrisbanes]

QQ: is there a performance impact from using opaque = false? It doesn't look like it from the PR

[elijah-semyonov]

@chrisbanes
Yes, it's marked in the documentation to the added configuration property.
There is an impact, since iOS can't draw perform direct-to-screen drawing when opaque is set to false.