LibGfx: Take filter by reference in Painter::blit_filtered()

This allows reusing the filter rather than moving it into blit_filtered.

Userland/Libraries/LibGfx/Painter.cpp

@@ -934,7 +934,7 @@ void Painter::blit_with_opacity(IntPoint position, Gfx::Bitmap const& source, In
     }
 }
 
-void Painter::blit_filtered(IntPoint position, Gfx::Bitmap const& source, IntRect const& src_rect, Function<Color(Color)> filter)
+void Painter::blit_filtered(IntPoint position, Gfx::Bitmap const& source, IntRect const& src_rect, Function<Color(Color)> const& filter)
 {
     VERIFY((source.scale() == 1 || source.scale() == scale()) && "blit_filtered only supports integer upsampling");
 

Userland/Libraries/LibGfx/Painter.h

@@ -84,7 +84,7 @@ public:
     void blit(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect, float opacity = 1.0f, bool apply_alpha = true);
     void blit_dimmed(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect);
     void blit_brightened(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect);
-    void blit_filtered(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect, Function<Color(Color)>);
+    void blit_filtered(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect, Function<Color(Color)> const&);
     void draw_tiled_bitmap(IntRect const& dst_rect, Gfx::Bitmap const&);
     void blit_offset(IntPoint, Gfx::Bitmap const&, IntRect const& src_rect, IntPoint);
     void blit_disabled(IntPoint, Gfx::Bitmap const&, IntRect const&, Palette const&);