PixelPaint: Keep track of and expose the type of the edited bitmap

This can be either the content bitmap or the mask bitmap.

Userland/Applications/PixelPaint/Layer.cpp

@@ -179,4 +179,25 @@ void Layer::create_mask()
     update_cached_bitmap();
 }
 
+Gfx::Bitmap& Layer::currently_edited_bitmap()
+{
+    switch (edit_mode()) {
+    case EditMode::Mask:
+        if (is_masked())
+            return *mask_bitmap();
+        [[fallthrough]];
+    case EditMode::Content:
+        return content_bitmap();
+    }
+    VERIFY_NOT_REACHED();
+}
+
+void Layer::set_edit_mode(Layer::EditMode mode)
+{
+    if (m_edit_mode == mode)
+        return;
+
+    m_edit_mode = mode;
+}
+
 }

Userland/Applications/PixelPaint/Layer.h

@@ -74,6 +74,16 @@ public:
 
     bool is_masked() { return !m_mask_bitmap.is_null(); }
 
+    enum class EditMode {
+        Content,
+        Mask,
+    };
+
+    EditMode edit_mode() { return m_edit_mode; }
+    void set_edit_mode(EditMode mode);
+
+    Gfx::Bitmap& currently_edited_bitmap();
+
 private:
     Layer(Image&, NonnullRefPtr<Gfx::Bitmap>, String name);
 
@@ -90,6 +100,8 @@ private:
 
     int m_opacity_percent { 100 };
 
+    EditMode m_edit_mode { EditMode::Content };
+
     void update_cached_bitmap();
 };