LibGfx: Correctly handle source rect offset in draw_scaled_bitmap

The do_draw_integer_scaled_bitmap() fastpath already handled this
correctly, but the arbitrary scale path did not.

Userland/Demos/LibGfxScaleDemo/main.cpp

@@ -100,6 +100,8 @@ void Canvas::draw(Gfx::Painter& painter)
 
     auto buggie = Gfx::Bitmap::load_from_file("/res/graphics/buggie.png");
     painter.blit({ 25, 39 }, *buggie, { 2, 30, 62, 20 });
+    painter.draw_scaled_bitmap({ 88, 39, 62 * 2, 20 * 2 }, *buggie, { 2, 30, 62, 20 });
+    painter.draw_scaled_bitmap({ 202, 39, 80, 40 }, *buggie, { 2, 30, 62, 20 });
 }
 
 int main(int argc, char** argv)

Userland/Libraries/LibGfx/Painter.cpp

@@ -843,7 +843,7 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, const IntRe
         for (int x = clipped_rect.left(); x <= clipped_rect.right(); ++x) {
             auto scaled_x = ((x - dst_rect.x()) * hscale) >> 16;
             auto scaled_y = ((y - dst_rect.y()) * vscale) >> 16;
-            auto src_pixel = get_pixel(source, scaled_x, scaled_y);
+            auto src_pixel = get_pixel(source, scaled_x + src_rect.left(), scaled_y + src_rect.top());
             if (has_opacity)
                 src_pixel.set_alpha(src_pixel.alpha() * opacity);
             if constexpr (has_alpha_channel) {