Explicitly wait for subpass buffers to be scheduled. (flutter#115)

This is pending the addition of explicit synchronization primitives to
Impeller. It is unclear what the addition of such primitives would mean
to the future OpenGL ES backend.

impeller/renderer/backend/metal/command_buffer_mtl.mm

@@ -173,6 +173,7 @@ static void LogMTLCommandBufferErrorIfPresent(id<MTLCommandBuffer> buffer) {
   }
 
   [buffer_ commit];
+  [buffer_ waitUntilScheduled];
   buffer_ = nil;
   return true;
 }