[ios] Release cached tiles on entering background.

Retain current render tiles for fast restart.
Waiting for a memory warning doesn't work because we can't make GL release calls once we're in the background.

platform/ios/src/MGLMapView.mm

@@ -1158,6 +1158,13 @@ public:
 - (void)sleepGL:(__unused NSNotification *)notification
 {
     MGLAssertIsMainThread();
+    
+    // Ideally we would wait until we actually received a memory warning but the bulk of the memory
+    // we have to release is tied up in GL buffers that we can't touch once we're in the background.
+    // Compromise position: release everything but currently rendering tiles
+    // A possible improvement would be to store a copy of the GL buffers that we could use to rapidly
+    // restart, but that we could also discard in response to a memory warning.
+    _rendererFrontend->reduceMemoryUse();
 
     if ( ! self.dormant)
     {