Fixed using image for tile 0 in scene

[jerone]

I couldn't use a zero indexed tile image for my scene.

[cykod]

This functionality was needed, but I added two methods in lieu of this (I usually want to use tile 0 to be not drawn instead of undefined as undefined looks bad in a json data file):

// Overload this method to draw tiles at frame 0 or not draw
// tiles at higher number frames
drawableTile: function(tileNum) {
  return tileNum > 0;
},

// Overload this method to control which tiles trigger a collision
// (defaults to all tiles > number 0)
collidableTile: function(tileNum) {
  return tileNum > 0;
},

If you inherit from Q.TileLayer you can control what tiles get drawn and what cause collisions.