LibSoftGPU: Also interpolate normal during triangle clipping

Userland/Libraries/LibSoftGPU/Clipper.cpp

@@ -48,6 +48,7 @@ Vertex Clipper::clip_intersection_point(const Vertex& p1, const Vertex& p2, Clip
     out.clip_coordinates = mix(p1.clip_coordinates, p2.clip_coordinates, a);
     out.color = mix(p1.color, p2.color, a);
     out.tex_coord = mix(p1.tex_coord, p2.tex_coord, a);
+    out.normal = mix(p1.normal, p2.normal, a);
     return out;
 }