tiled backgrounds no longer has strange off-by-one pixel errors

Demos/PaintTest/main.cpp

@@ -0,0 +1,50 @@
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GWindow.h>
+#include <LibGUI/GWidget.h>
+#include <LibGUI/GPainter.h>
+#include <SharedGraphics/PNGLoader.h>
+
+class TestWidget final : public GWidget {
+public:
+    TestWidget(GWidget* parent) : GWidget(parent) { }
+    virtual ~TestWidget() override { }
+
+    void set_bitmap(RetainPtr<GraphicsBitmap>&& bitmap)
+    {
+        m_bitmap = move(bitmap);
+        update();
+    }
+
+private:
+    virtual void paint_event(GPaintEvent&) override
+    {
+        GPainter painter(*this);
+
+        painter.fill_rect(rect(), Color::LightGray);
+
+        painter.blit_tiled({ 0, 0, 160, 160 }, *m_bitmap, m_bitmap->rect());
+
+        painter.add_clip_rect({ 50, 50, 115, 95 });
+        painter.blit_tiled({ 160, 160, 160, 160 }, *m_bitmap, m_bitmap->rect());
+    }
+
+    RetainPtr<GraphicsBitmap> m_bitmap;
+};
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+
+    auto* window = new GWindow;
+    window->set_rect(100, 100, 400, 400);
+    window->set_title("Paint test");
+
+    auto* test_widget = new TestWidget(nullptr);
+    window->set_main_widget(test_widget);
+
+    test_widget->set_bitmap(load_png("/res/icons/gear16.png"));
+
+    window->show();
+
+    return app.exec();
+}