add compositeOperation argument to Surface.blit

pretty much what pygames special_flags argument does: it affects how the two surfaces are composited. compare:

 * http://www.pygame.org/docs/ref/surface.html#Surface.fill
 * http://dev.w3.org/html5/2dcontext/#dom-context-2d-globalcompositeoperation

lib/gamejs.js

@@ -573,10 +573,10 @@ Surface.prototype._smooth = function() {
  *            is not supplied the blit happens at [0,0].
  * @param {gamesjs.Rect|Array} area the Area from the passed Surface which
  *            should be blitted onto this Surface.
- * @param {Number} [special_flags] FIXME add special flags for composite operations
+ * @param {Number} compositionOperation how the source and target surfaces are composited together; one of: source-atop, source-in, source-out, source-over (default), destination-atop, destination-in, destination-out, destination-over, lighter, copy, xor; for an explanation of these values see: http://dev.w3.org/html5/2dcontext/#dom-context-2d-globalcompositeoperation
  * @returns {gamejs.Rect} Rect actually repainted
  */
-Surface.prototype.blit = function(src, dest, area, special_flags) {
+Surface.prototype.blit = function(src, dest, area, compositeOperation) {
 
    var rDest, rArea;
 
@@ -594,6 +594,7 @@ Surface.prototype.blit = function(src, dest, area, special_flags) {
     } else {
       rDest = new Rect([0,0], src.getSize());
     }
+   compositeOperation = compositeOperation || 'source-over';
 
    // area within src to be drawn
    if (area instanceof Rect) {
@@ -610,6 +611,7 @@ Surface.prototype.blit = function(src, dest, area, special_flags) {
    }
 
    this.context.save();
+   this.context.globalCompositeOperation = compositeOperation;
    // first translate, then rotate
    var m = matrix.translate(matrix.identity(), rDest.left, rDest.top);
    m = matrix.multiply(m, src._matrix);