Fix AsyncImagePainter not updating placeholder/error/fallback when transform changes

[shamana-s]

Summary

Fixes an issue where updating the placeholder, error, or fallback painter after AsyncImagePainter has already entered the corresponding state does not update the displayed painter.

The root cause is that the transform callback is updated during recomposition, but it is only applied when a new image loading state is emitted. If only the transform changes, the current state is never re-transformed.

This change stores the latest untransformed state and reapplies the transform whenever the transform callback changes, without restarting the image request.

Reproduction

var fallback by remember {
    mutableStateOf<Painter>(ColorPainter(Color.Red))
}

LaunchedEffect(Unit) {
    delay(2.seconds)
    fallback = ColorPainter(Color.Blue)
}

AsyncImage(
    model = null,
    fallback = fallback,
    contentDescription = null,
)

Before

The displayed fallback painter remained red indefinitely.

After

The displayed fallback painter updates to blue immediately after recomposition.

Visual demonstration

The video below shows the fallback painter updating from red to blue after the fallback parameter changes, without restarting the image request.

Screen_recording_20260716_144515.mp4

Fixes #1908

Related: #3477