PixelPaint: Editing mask optimization

This patch introduces a new function "Layer::editin_mask_bounding_rect"
that is used within the LevelsDialog, Luminosity and Colormasking to
process only the area where a mask was applied. Therefore we can
greatly reduce the amount of processed pixels if only a small portion
of the image was masked.

Userland/Applications/PixelPaint/LevelsDialog.h

@@ -28,6 +28,7 @@ private:
     RefPtr<GUI::ValueSlider> m_gamma_slider = { nullptr };
     bool m_did_change = false;
     int m_precomputed_color_correction[256];
+    Optional<Gfx::IntRect> m_masked_area;
 
     ErrorOr<void> ensure_reference_bitmap();
     void generate_new_image();