Fix "Unsupported concurrent change during composition"

[igordmn]

Fixes https://youtrack.jetbrains.com/issue/CMP-6729/iOS-Runtime-crash.-Unsupported-concurrent-change-during-composition

The issue was because of this concurrent calls:

performRecompose()
  val snapshot = Snapshot.takeMutableSnapshot()
    Snapshot.sendApplyNotifications() // update the Global snapshot, called because we need to pass it fresh to the local Recompose snapshot
      drawState.value = Unit // called because some other state captured in `draw` was changed
  snapshot.enter {
    ...
    Snapshot.sendApplyNotifications() // it is valid to call it anywhere
      drawState.value = Unit // called because some other state state captured in `draw` was changed again
  }
  snapshot.apply() // fails with "Unsupported concurrent change during composition" because it can't merge to state changes

Regression after #1574

A synthetic reproducer of the issue for history (skip it if details are already understood):

import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.neverEqualPolicy
import androidx.compose.runtime.snapshots.Snapshot
import androidx.compose.runtime.snapshots.SnapshotStateObserver

fun main() {
    val drawState = mutableStateOf(Unit, neverEqualPolicy())

    val observer = SnapshotStateObserver { it.invoke() }
    observer.start()

    fun draw() {
        observer.observeReads(Unit, {
            println("invalidate draw (drawState.value = Unit)")
            drawState.value = Unit
        }) {
            println("\ndraw")
            drawState.value
        }
    }

    fun externalChange() {
        println("\nexternalChange")
        drawState.value = Unit
        Snapshot.sendApplyNotifications()
    }

    fun composition() {
        println("\ncomposition BEFORE")
        Snapshot.takeMutableSnapshot().apply {
            enter {
                println("composition BEGIN")
                println("sendApplyNotifications")
                Snapshot.sendApplyNotifications()
                println("composition END")
            }
            println("applying composition state to global state")
            if (!apply().succeeded) {
                throw RuntimeException("Composition apply failed")
            }
        }
    }

    // frame 0
    draw()

    externalChange()

    // frame 1
    composition()
}

Doesn't represent real case, as we capture and change drawState of the same layer. In a real case layer captures only children drawState. Still it shows similar chain of calls

Output:

draw

externalChange
invalidate draw (drawState.value = Unit)

composition BEFORE
invalidate draw (drawState.value = Unit)
composition BEGIN
sendApplyNotifications
invalidate draw (drawState.value = Unit)
composition END
applying composition state to global state
Exception in thread "main" java.lang.RuntimeException: Composition apply failed
    at MainKt.main$composition(main.kt:39)
    at MainKt.main(main.kt:50)
    at MainKt.main(main.kt)