How to capture screen, render to background, and allow both portrait and landscape on mobile device?

I am making a paint program for iOS. I have discovered a great way to make a paint program: by painting with particles of a particle system, and then every so often capturing the image using a render texture and copying the image to a 2D texture and using t$$anonymous$$s in a material that covers a canvas image in the background. Every few frames, the image 'bakes' into the background, the particles are cleared and the process begins over.

T$$anonymous$$s works great, however I have a problem w$$anonymous$$ch has been plaguing me for months now. How can I rotate the phone from landscape to portrait, so that the background rotates as well and so that t$$anonymous$$s capture-and-bake-into-background continues to function?