LibWeb: Make CSS background image painting respect destination origin and transparency

It was previously using draw_tiled_bitmap, which always aligns the
tiles with the global origin and does not respect the alpha of the
source. Switch to a new Painter::blit_tiled helper which uses
Painter::blit under the hood, which has more correct behavior.

Userland/Libraries/LibGfx/Painter.cpp

@@ -1726,4 +1726,26 @@ void Painter::blit_disabled(const IntPoint& location, const Gfx::Bitmap& bitmap,
     });
 }
 
+void Painter::blit_tiled(const IntRect& dst_rect, const Gfx::Bitmap& bitmap, const IntRect& rect)
+{
+    auto tile_width = rect.width();
+    auto tile_height = rect.height();
+    auto dst_right = dst_rect.right();
+    auto dst_bottom = dst_rect.bottom();
+    for (int tile_y = dst_rect.top(); tile_y < dst_bottom; tile_y += tile_height) {
+        for (int tile_x = dst_rect.left(); tile_x < dst_right; tile_x += tile_width) {
+            IntRect tile_src_rect = rect;
+            auto tile_x_overflow = tile_x + tile_width - dst_right;
+            if (tile_x_overflow > 0) {
+                tile_src_rect.set_width(tile_width - tile_x_overflow);
+            }
+            auto tile_y_overflow = tile_y + tile_height - dst_bottom;
+            if (tile_y_overflow > 0) {
+                tile_src_rect.set_height(tile_height - tile_y_overflow);
+            }
+            blit(IntPoint(tile_x, tile_y), bitmap, tile_src_rect);
+        }
+    }
+}
+
 }

Userland/Libraries/LibGfx/Painter.h

@@ -77,6 +77,7 @@ public:
     void draw_tiled_bitmap(const IntRect& dst_rect, const Gfx::Bitmap&);
     void blit_offset(const IntPoint&, const Gfx::Bitmap&, const IntRect& src_rect, const IntPoint&);
     void blit_disabled(const IntPoint&, const Gfx::Bitmap&, const IntRect&, const Palette&);
+    void blit_tiled(const IntRect&, const Gfx::Bitmap&, const IntRect& src_rect);
     void draw_text(const IntRect&, const StringView&, const Font&, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);
     void draw_text(const IntRect&, const StringView&, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);
     void draw_text(const IntRect&, const Utf32View&, const Font&, TextAlignment = TextAlignment::TopLeft, Color = Color::Black, TextElision = TextElision::None);

Userland/Libraries/LibWeb/Layout/Box.cpp

@@ -46,10 +46,11 @@ void Box::paint(PaintContext& context, PaintPhase phase)
     auto padded_rect = this->padded_rect();
 
     if (phase == PaintPhase::Background && !is_body()) {
-        context.painter().fill_rect(enclosing_int_rect(padded_rect), computed_values().background_color());
-
-        if (background_image() && background_image()->bitmap())
-            context.painter().draw_tiled_bitmap(enclosing_int_rect(padded_rect), *background_image()->bitmap());
+        auto background_rect = enclosing_int_rect(padded_rect);
+        context.painter().fill_rect(background_rect, computed_values().background_color());
+        if (background_image() && background_image()->bitmap()) {
+            context.painter().blit_tiled(background_rect, *background_image()->bitmap(), background_image()->bitmap()->rect());
+        }
     }
 
     if (phase == PaintPhase::Border) {