Turn off front-camera workaround on iOS 6 or later

As discussed in #462, there are various problems with the workaround for corrupt images on iOS 5. Given that iOS 5 is quite legacy now, I think it's a reasonable solution to turn off that workaround if GPUImage is running on iOS 6 or later.

This could of course be adjusted if the precise version of the OS required for the workaround is not 5.X.
Fixed a problem where these 2 filters would not correctly set the frameSize through the forceProcessingAtSize: method
Merge pull request #1120 from paolonl/master

Fixed GPUImageAmatorkaFilter and GPUImageMissEtikateFilter not processing images at a desired size
Merge pull request #1103 from karlvr/front-camera-ios5-bug

Turn off front-camera workaround on iOS 6 or later
Fixed a Mac build error introduced by a recent pull request.