P: SDK: Provide a method to flip photos

The SDK provides no way for a plugin to flip a photo horizontally or vertically. There is photo:rotateRight() and photo:rotateLeft() but no analogous methods for flipping.

My Any Crop plugin would use this to provide a command to restore a photo to its original orientation.  It has a command for doing that with rotation but not flipping.

My Copy Settings plugin's Extract Masks command would use this to extract the masks of flipped photos.