gl_rasterizer: Implement constant vertex attributes

Credits go to gdkchan from Ryujinx for finding constant attributes are
used in retail games.

src/video_core/engines/maxwell_3d.h

@@ -303,6 +303,10 @@ public:
                 return (type == Type::SignedNorm) || (type == Type::UnsignedNorm);
             }
 
+            bool IsConstant() const {
+                return constant;
+            }
+
             bool IsValid() const {
                 return size != Size::Invalid;
             }