Implemented eraser tool. Must move and resize layers at the same time.

Signed-off-by: npalomba <nicola.palomba@studenti.galileilivorno.gov.it>

js/_checkerboard.js

@@ -6,6 +6,8 @@ var nSquaresFilled = 0;
  */
 
 function fillCheckerboard() {
+    var context = checkerBoard.context;
+
     for (var i=0; i<canvasSize[0] / checkerBoardSquareSize; i++) {
         nSquaresFilled = 0;