PixelPaint: Implement Merge Active Layer Up

Implements 'Merge Active Layer Up' and adds it to the 'Layer' menu.

Userland/Applications/PixelPaint/Image.cpp

@@ -369,6 +369,23 @@ void Image::merge_visible_layers()
     }
 }
 
+void Image::merge_active_layer_up(Layer& layer)
+{
+    if (m_layers.size() < 2)
+        return;
+    size_t layer_index = this->index_of(layer);
+    if ((layer_index + 1) == m_layers.size()) {
+        dbgln("Cannot merge layer up: layer is already at the top");
+        return; // FIXME: Notify user of error properly.
+    }
+
+    auto& layer_above = m_layers.at(layer_index + 1);
+    GUI::Painter painter(layer_above.bitmap());
+    painter.draw_scaled_bitmap(rect(), layer.bitmap(), layer.rect(), (float)layer.opacity_percent() / 100.0f);
+    remove_layer(layer);
+    select_layer(&layer_above);
+}
+
 void Image::merge_active_layer_down(Layer& layer)
 {
     if (m_layers.size() < 2)

Userland/Applications/PixelPaint/Image.h

@@ -81,6 +81,7 @@ public:
     void select_layer(Layer*);
     void flatten_all_layers();
     void merge_visible_layers();
+    void merge_active_layer_up(Layer& layer);
     void merge_active_layer_down(Layer& layer);
 
     void add_client(ImageClient&);

Userland/Applications/PixelPaint/MainWidget.cpp

@@ -591,6 +591,17 @@ void MainWidget::initialize_menubar(GUI::Window& window)
             editor->did_complete_action();
         }));
 
+    m_layer_menu->add_action(GUI::Action::create(
+        "Merge &Active Layer Up", [&](auto&) {
+            auto* editor = current_image_editor();
+            VERIFY(editor);
+            auto active_layer = editor->active_layer();
+            if (!active_layer)
+                return;
+            editor->image().merge_active_layer_up(*active_layer);
+            editor->did_complete_action();
+        }));
+
     m_layer_menu->add_action(GUI::Action::create(
         "M&erge Active Layer Down", { Mod_Ctrl, Key_E }, [&](auto&) {
             auto* editor = current_image_editor();