Extract canvas clear command planning

src/app_layout.cpp

@@ -10,6 +10,7 @@
 #include "app_core/brush_ui.h"
 #include "app_core/canvas_tool_ui.h"
 #include "app_core/document_layer.h"
+#include "app_core/document_canvas.h"
 #include "app_core/app_status.h"
 #include "app_core/history_ui.h"
 #include "settings.h"
@@ -149,8 +150,13 @@ void App::init_toolbar_main()
     {
         button->on_click = [this](Node*) {
             //exit(0); 
-            if (canvas)
-                canvas->m_canvas->clear({ 0, 0, 0, 0 });
+            const auto plan = pp::app::plan_document_canvas_clear(static_cast<bool>(canvas));
+            if (plan && plan.value().clears_canvas)
+                canvas->m_canvas->clear({
+                    plan.value().r,
+                    plan.value().g,
+                    plan.value().b,
+                    plan.value().a });
         };
     }
     if (auto* button = layout[main_id]->find<NodeButton>("btn-popup"))