Shape the surface behind an AlertDialog according to the dialog's shape.

[m-sasha]

Proposed Changes

• Pass the shape defining the AlertDialog to the AlertDialogProvider.
• Change PopupAlertDialogProvider to pass the shape to the Surface that displays the elevation shadow, in order to draw the shadow with the correct shape.
• Change PopupAlertDialogProvider to use a transparent background for the Surface that displays the elevation shadow. Otherwise the edges of its background can still show through the dialog content's shape.
  AlertDialogContent uses its own Surface at the root, so unless the user passes a translucent backgroundColor to AlertDialog, this change should not be visible.
• Likewise change UndecoratedWindowAlertDialogProvider to use CoreDialog(transparent=true).

Note that this adds a new function to the (public) AlertDialogProvider interface, but the new function has a default implementation that delegates to the old function. I think this provides backwards compatibility.

Reproducer:

@OptIn(ExperimentalMaterialApi::class)
fun main() = singleWindowApplication(
    state = WindowState(width = 300.dp, height = 300.dp)
) {
    Box{
        MaterialTheme {
            AlertDialog(
                onDismissRequest = { },
                buttons = {},
                text = { Text("AlertDialog") },
                shape = RoundedCornerShape(20.dp),
                backgroundColor = Color.Yellow,
            )
        }
    }
}

Before:

After:

Testing

Test: Manual testing using the code above.

Issues Fixed

Fixes: JetBrains/compose-multiplatform#1269