Fix iOS crash when multiple platform views are in the scene (flutter#…

…13449)

Having 2 or more platform views simultaneously in the layer tree was crashing immediately on iOS with GL backend.

This regressed in flutter#11070 which passed gl_context to a function in a loop using std::move (which meant on the second iteration the caller is no longer the owner of the field).

I added a scenarios_app test, though this test doesn't run on a physical device on CI so it would have only caught the problem when running locally (flutter/flutter#43852).

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -375,7 +375,7 @@
   for (int64_t view_id : composition_order_) {
     auto contextSwitch = ConstructRendererContextSwitchIfAvailable();
 
-    EnsureOverlayInitialized(view_id, std::move(gl_context), gr_context);
+    EnsureOverlayInitialized(view_id, gl_context, gr_context);
     auto frame = overlays_[view_id]->surface->AcquireFrame(frame_size_);
     SkCanvas* canvas = frame->SkiaCanvas();
     canvas->drawPicture(picture_recorders_[view_id]->finishRecordingAsPicture());