[iOS] increase backdrop cached task limit. (#164036)

Partial workaround for flutter/flutter#161142
. Since this was an arbitrary number anyway we can increase it a bit.

Author: Jonah Williams

engine/src/flutter/impeller/renderer/context.h

@@ -57,7 +57,7 @@ class Context {
   /// This number was arbitrarily chosen. The idea is that this is a somewhat
   /// rare situation where tasks happen to get executed in that tiny amount of
   /// time while an app is being backgrounded but still executing.
-  static constexpr int32_t kMaxTasksAwaitingGPU = 10;
+  static constexpr int32_t kMaxTasksAwaitingGPU = 64;
 
   //----------------------------------------------------------------------------
   /// @brief      Destroys an Impeller context.