Offset parameter for particle effects

Would allow a particle effect - particles and all - to be moved around the screen. An example of this being useful would be a scrolling game in which an explosion takes place in a particular location in the level whilst the player is moving - currently, the source could move with the level but the particles would stay in place, making the explosion appear to be blown in the direction of the camera's movement. This has also become an issue in the early tests created for a particle effect editor, as resizing the window makes the source move but not the particles, with slightly odd results.