Add depth offset

[Annoraaq]

A custom y-offset for the sprite/container depth. In GridEngine the depth sorting of characters depends on their character layer and on their y pixel position. By setting a depthOffset you can change the y pixel position for the depth sorting without changing the actual y pixel position.

For example: Consider two characters A and B that are on the same character layer. If char A is on y pixel position 100 and char B is on y pixel position 120, then char B would be rendered in front of char A. If you set depthOffset = -50 for char B then char A would be rendered on top of char B instead (because the depth relevant y pos of char B is 120 - 50 = 80 and that of char A is 100).