LibGfx: Implement SmoothPixels scaling mode

If you wanted to upscale an image, you had two options:
- use Nearest Neighbor: it's probably a good choice. The image stays
  sharp.. unless you aren't using integer scales.
- use Bilinear blending, but this on the other hand, doesn't handle
  upscaling well. It just blurs everything.

But what if we could take the best of both of them and make the image
sharp on integers and just blur it a little when needed?

Well, there's Smooth Pixels!

This mode is similar to the Bilinear Blend, with the main difference
is that the blend ratio is multiplied by the current scale, so the blur
on corners can be only 1px wide.

From my testing this mode doesn't handles downscaling as good as the
Bilinear blending though.

Userland/Libraries/LibGfx/Painter.cpp

@@ -1100,7 +1100,7 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
     if (clipped_src_rect.is_empty())
         return;
 
-    if constexpr (scaling_mode == Painter::ScalingMode::NearestNeighbor) {
+    if constexpr (scaling_mode == Painter::ScalingMode::NearestNeighbor || scaling_mode == Painter::ScalingMode::SmoothPixels) {
         if (dst_rect == clipped_rect && int_src_rect == src_rect && !(dst_rect.width() % int_src_rect.width()) && !(dst_rect.height() % int_src_rect.height())) {
             int hfactor = dst_rect.width() / int_src_rect.width();
             int vfactor = dst_rect.height() / int_src_rect.height();
@@ -1155,6 +1155,27 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
                 auto bottom = bottom_left.interpolate(bottom_right, x_ratio);
 
                 src_pixel = top.interpolate(bottom, y_ratio);
+            } else if constexpr (scaling_mode == Painter::ScalingMode::SmoothPixels) {
+                auto scaled_x1 = clamp(desired_x >> 32, clipped_src_rect.left(), clipped_src_rect.right());
+                auto scaled_x0 = clamp(scaled_x1 - 1, clipped_src_rect.left(), clipped_src_rect.right());
+                auto scaled_y1 = clamp(desired_y >> 32, clipped_src_rect.top(), clipped_src_rect.bottom());
+                auto scaled_y0 = clamp(scaled_y1 - 1, clipped_src_rect.top(), clipped_src_rect.bottom());
+
+                float x_ratio = (desired_x & fractional_mask) / (float)shift;
+                float y_ratio = (desired_y & fractional_mask) / (float)shift;
+
+                float scaled_x_ratio = clamp(x_ratio * dst_rect.width() / (float)src_rect.width(), 0.0f, 1.0f);
+                float scaled_y_ratio = clamp(y_ratio * dst_rect.height() / (float)src_rect.height(), 0.0f, 1.0f);
+
+                auto top_left = get_pixel(source, scaled_x0, scaled_y0);
+                auto top_right = get_pixel(source, scaled_x1, scaled_y0);
+                auto bottom_left = get_pixel(source, scaled_x0, scaled_y1);
+                auto bottom_right = get_pixel(source, scaled_x1, scaled_y1);
+
+                auto top = top_left.interpolate(top_right, scaled_x_ratio);
+                auto bottom = bottom_left.interpolate(bottom_right, scaled_x_ratio);
+
+                src_pixel = top.interpolate(bottom, scaled_y_ratio);
             } else {
                 auto scaled_x = clamp(desired_x >> 32, clipped_src_rect.left(), clipped_src_rect.right());
                 auto scaled_y = clamp(desired_y >> 32, clipped_src_rect.top(), clipped_src_rect.bottom());
@@ -1179,6 +1200,9 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
     case Painter::ScalingMode::NearestNeighbor:
         do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::NearestNeighbor>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
         break;
+    case Painter::ScalingMode::SmoothPixels:
+        do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::SmoothPixels>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
+        break;
     case Painter::ScalingMode::BilinearBlend:
         do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::BilinearBlend>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
         break;

Userland/Libraries/LibGfx/Painter.h

@@ -36,6 +36,7 @@ public:
 
     enum class ScalingMode {
         NearestNeighbor,
+        SmoothPixels,
         BilinearBlend,
     };