Fix build errors

When I first cloned GPUImage and attempted to build the "FilterShowcase" example, I received several build errors related to a missing `kGPUImageRotateRightFlipHorizontal` type. I added this to the `GPUImageRotationMode` enum in GPUImageContext.h and then was able to build.

Very new to this project, so apologies if I'm missing something here ... this is just what I had to do to get it to build out of the box.