Fix to ISurface and NESBank .getColorIndex() method's return value.

app/js/ifaces/ISurface.js

@@ -50,7 +50,7 @@ export default class ISurface{
   }
 
   getColorIndex(x, y){
-    return 0;
+    return {pi:-1, ci:-1};
   }
 
   setColorIndex(x, y, ci, pi){

app/js/models/NESBank.js

@@ -194,6 +194,9 @@ export default class NESBank extends ISurface{
   }
 
   getColorIndex(x, y){
+    if (x < 0 || x >= 256 || y < 0 || y >= 128)
+      return {pi: -1, ci:this.__default_pi[4]};
+
     var res = LRIdx2TileIdxCo((y*256)+x);
     var list = (res.lid === 0) ? this.__LP : this.__RP; 
     return {