PixelPaint: Add method to merge visible layers

This adds a function that merges all visible layers to one, ignoring the
invisible.

Userland/Applications/PixelPaint/Image.cpp

@@ -384,6 +384,34 @@ void Image::flatten_all_layers()
     select_layer(&bottom_layer);
 }
 
+void Image::merge_visible_layers()
+{
+    if (m_layers.size() < 2)
+        return;
+
+    size_t index = 0;
+
+    while (index < m_layers.size()) {
+        if (m_layers.at(index).is_visible()) {
+            auto& bottom_layer = m_layers.at(index);
+            GUI::Painter painter(bottom_layer.bitmap());
+            paint_into(painter, { 0, 0, m_size.width(), m_size.height() });
+            select_layer(&bottom_layer);
+            index++;
+            break;
+        }
+        index++;
+    }
+    while (index < m_layers.size()) {
+        if (m_layers.at(index).is_visible()) {
+            auto& layer = m_layers.at(index);
+            remove_layer(layer);
+        } else {
+            index++;
+        }
+    }
+}
+
 void Image::select_layer(Layer* layer)
 {
     for (auto* client : m_clients)

Userland/Applications/PixelPaint/Image.h

@@ -69,6 +69,7 @@ public:
     void remove_layer(Layer&);
     void select_layer(Layer*);
     void flatten_all_layers();
+    void merge_visible_layers();
 
     void add_client(ImageClient&);
     void remove_client(ImageClient&);

Userland/Applications/PixelPaint/main.cpp

@@ -419,6 +419,16 @@ int main(int argc, char** argv)
         },
         window));
 
+    layer_menu.add_action(GUI::Action::create(
+        "&Merge Visible", { Mod_Ctrl, Key_M }, [&](auto&) {
+            auto* editor = current_image_editor();
+            if (!editor)
+                return;
+            editor->image().merge_visible_layers();
+            editor->did_complete_action();
+        },
+        window));
+
     auto& filter_menu = menubar->add_menu("&Filter");
     auto& spatial_filters_menu = filter_menu.add_submenu("&Spatial");