LibSoftGPU: Take sample from pixel center

This adds a half pixel offset to the edge value calculation in order to
sample the triangle at pixel centers. This is in line with actual OpenGL
rasterization rules and generates correctly interpolated vertex
attributes including texture coordinates.

Userland/Libraries/LibSoftGPU/Device.cpp

@@ -259,6 +259,11 @@ void Device::rasterize_triangle(const Triangle& triangle)
     int const render_bounds_top = render_bounds.y();
     int const render_bounds_bottom = render_bounds.y() + render_bounds.height();
 
+    auto const half_pixel_offset = Vector2<i32x4> {
+        expand4(subpixel_factor / 2),
+        expand4(subpixel_factor / 2),
+    };
+
     // Iterate over all blocks within the bounds of the triangle
     for (int by = by0; by < by1; by += 2) {
         for (int bx = bx0; bx < bx1; bx += 2) {
@@ -270,7 +275,7 @@ void Device::rasterize_triangle(const Triangle& triangle)
                 i32x4 { by, by, by + 1, by + 1 },
             };
 
-            auto edge_values = calculate_edge_values4(quad.screen_coordinates * subpixel_factor);
+            auto edge_values = calculate_edge_values4(quad.screen_coordinates * subpixel_factor + half_pixel_offset);
 
             // Generate triangle coverage mask
             quad.mask = test_point4(edge_values);