Fixes an issue with Coil's public composables where its memory cache check could miss the first frame due to dispatching. This case occurs with Paparazzi/Roborazzi/AndroidX screenshot tests and is due to the dispatcher provided by rememberCoroutineScope() either not supporting immediate dispatching, Dispatchers.Main.immediate being unavailable, or Dispatchers.Main.immediate using a different underlying thread.

To fix this I tried a couple things:

• Using CoroutineStart.UNDISPATCHED with a StateFlow. This works for initial composition, but does not resolve synchronously from the memory cache on subsequent requests (e.g. when the AsyncImage(model) changes).
• Attempting to create a CoroutineDispatcher that re-enables dispatching after the first time the context's CoroutineDispatcher changes. As far as I can tell this isn't possible at the moment with CoroutineInterceptor's hooks.

I settled on using a custom CoroutineDispatcher that acts like Dispatchers.Unconfined until immediately after the memory cache check. This is safe for Coil's internals as it's all thread safe. Aside from fixing the above issue this has a number of benefits:

• This ensures we have consistent behaviour in all environments instead of falling back to Dispatchers.Unconfined if Dispatchers.Main.immediate is unavailable.
• kotlinx-coroutines-swing no longer needs to be added manually as a dependency on JVM.