LibWeb: Fix some `conic-gradient()` painting issues

This makes the center position the center of the pixel rather than
the top left corner (which fixes some small artifacts on a few
gradients).

This also now floors the angle used to sample from the gradient line,
this avoids the colors diverging the further away from the center you
get (which is noticeable on hard-edge gradients).

Userland/Libraries/LibWeb/Painting/GradientPainting.cpp

@@ -273,10 +273,13 @@ void paint_conic_gradient(PaintContext& context, Gfx::IntRect const& gradient_re
     // FIXME: Do we need/want sub-degree accuracy for the gradient line?
     GradientLine gradient_line(360, data.color_stops);
     float start_angle = (360.0f - data.start_angle) + 90.0f;
+    // Translate position/center to the center of the pixel (avoids some funky painting)
+    auto center_point = Gfx::FloatPoint { position }.translated(0.5, 0.5);
     gradient_line.paint_into_rect(context.painter(), gradient_rect, [&](int x, int y) {
-        auto point = Gfx::IntPoint { x, y } - position;
+        auto point = Gfx::FloatPoint { x, y } - center_point;
         // FIXME: We could probably get away with some approximation here:
-        return fmod((AK::atan2(float(point.y()), float(point.x())) * 180.0f / AK::Pi<float> + 360.0f + start_angle), 360.0f);
+        // Note: We need too floor the angle here or the colors will start to diverge as you get further from the center.
+        return floor(fmod((AK::atan2(point.y(), point.x()) * 180.0f / AK::Pi<float> + 360.0f + start_angle), 360.0f));
     });
 }