BUGFIX: Video was not being XORed but just set.

gemma/src/chip8/video.rs

@@ -55,7 +55,7 @@ impl Chip8Video {
         } else {
             trace!("NOT TOGGLING");
         }
-        self.memory[effective_address as usize] = new_value;
+        self.memory[effective_address as usize] = new_value ^ old_value;
         self.to_owned()
     }