fix: RequestBuilder.submitAsync() throwing an Exception if the Glide request is restarted (#4569)

This is the third attempt to fix `RequestBuilder.submitAsync()`. For the
rationale, see the comments of #4436.

We now clear the continuation reference after resuming it, to make sure
that:
1) It will only be resumed once
2) It will not leak the coroutine when Glide keeps the `Request` around.

app/src/main/java/com/keylesspalace/tusky/util/GlideExtensions.kt

@@ -5,6 +5,7 @@ import com.bumptech.glide.load.DataSource
 import com.bumptech.glide.load.engine.GlideException
 import com.bumptech.glide.request.RequestListener
 import com.bumptech.glide.request.target.Target
+import kotlin.coroutines.Continuation
 import kotlin.coroutines.resume
 import kotlin.coroutines.resumeWithException
 import kotlinx.coroutines.suspendCancellableCoroutine
@@ -17,32 +18,41 @@ suspend fun <R> RequestBuilder<R>.submitAsync(
     height: Int = Target.SIZE_ORIGINAL
 ): R {
     return suspendCancellableCoroutine { continuation ->
-        val target = addListener(
-            object : RequestListener<R> {
-                override fun onLoadFailed(
-                    e: GlideException?,
-                    model: Any?,
-                    target: Target<R>,
-                    isFirstResource: Boolean
-                ): Boolean {
-                    continuation.resumeWithException(e ?: GlideException("Image loading failed"))
-                    return false
-                }
+        val target = addListener(ContinuationRequestListener(continuation))
+            .submit(width, height)
+        continuation.invokeOnCancellation { target.cancel(true) }
+    }
+}
+
+private class ContinuationRequestListener<R>(continuation: Continuation<R>) : RequestListener<R> {
+    private var continuation: Continuation<R>? = continuation
 
-                override fun onResourceReady(
-                    resource: R & Any,
-                    model: Any,
-                    target: Target<R>?,
-                    dataSource: DataSource,
-                    isFirstResource: Boolean
-                ): Boolean {
-                    if (target?.request?.isComplete == true) {
-                        continuation.resume(resource)
-                    }
-                    return false
-                }
+    override fun onLoadFailed(
+        e: GlideException?,
+        model: Any?,
+        target: Target<R>,
+        isFirstResource: Boolean
+    ): Boolean {
+        continuation?.let {
+            continuation = null
+            it.resumeWithException(e ?: GlideException("Image loading failed"))
+        }
+        return false
+    }
+
+    override fun onResourceReady(
+        resource: R & Any,
+        model: Any,
+        target: Target<R>?,
+        dataSource: DataSource,
+        isFirstResource: Boolean
+    ): Boolean {
+        continuation?.let {
+            if (target?.request?.isComplete == true) {
+                continuation = null
+                it.resume(resource)
             }
-        ).submit(width, height)
-        continuation.invokeOnCancellation { target.cancel(true) }
+        }
+        return false
     }
 }