[Linux][FlView] fix rendering on startup when shown after plugin registration

[jpnurmi]

This PR contains the engine-side changes required for changing the Linux app template to show the window after registering plugins (flutter/flutter#118269).

Currently, FlView assumes that it gets realized before its size gets allocated. If we change the order in the app template, the view gets added to a layout first and then realized. Make sure to call handle_geometry_changed() from both and check the pre-conditions for fl_renderer_wait_for_frame() to ensure that the first frame is rendered regardless of the order of size-allocate and realize.

This makes it possible to change my_application.cc as follows:

diff --git a/linux/my_application.cc b/linux/my_application.cc
index d9eae08..59af183 100644
--- a/linux/my_application.cc
+++ b/linux/my_application.cc
@@ -48,7 +48,6 @@ static void my_application_activate(GApplication* application) {
   }
 
   gtk_window_set_default_size(window, 1280, 720);
-  gtk_widget_show(GTK_WIDGET(window));
 
   g_autoptr(FlDartProject) project = fl_dart_project_new();
   fl_dart_project_set_dart_entrypoint_arguments(project, self->dart_entrypoint_arguments);
@@ -59,6 +58,7 @@ static void my_application_activate(GApplication* application) {
 
   fl_register_plugins(FL_PLUGIN_REGISTRY(view));
 
+  gtk_widget_show(GTK_WIDGET(window));
   gtk_widget_grab_focus(GTK_WIDGET(view));
 }
 

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

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[robert-ancell]

LGTM

[cbracken]

Thanks @jpnurmi !