Pica/TextureEnvironment: Add support for the MAD-like texture combiners and clean up texture environment logic.

src/video_core/pica.h

@@ -266,6 +266,9 @@ struct Regs {
             AddSigned       = 3,
             Lerp            = 4,
             Subtract        = 5,
+
+            MultiplyThenAdd = 8,
+            AddThenMultiply = 9,
         };
 
         union {