On start of layout: set instance variable originalX to background.X
On start of layout: set instance variable originalY to background.Y
Every tick: Background: move at angle self.angle
Compare two values:
distance(background.x,background.y,
background.originalX,background.originalY) >= tilewidth
-->
background: set position to self.originalX,self originalY.
This will let you scroll at any angle you want to tilt the background, and also take care of the tile discontinuity. It is assumed you will used a tiledbackground object. 'tilewidth' is the width of the source image, so it can loop back on itself seamlessly.