iOS Video Grabber changes for Simulator



- Removes preprocessor for arm architecture
- When targeting a iOS Simulator device will throw a #warning to inform
the developer that the grabber will not function in the simulator
- Initialises some variables (width/height/currentframe) to default of
0 in case init fails (which it will with simulator target)
- In realtime will throw a ```OF_LOG_WARNING``` when running on the simulator
at time of init to inform the developer this will not function
correctly.
- Allows developer to compile and test code even with a project using
the iOS Video Grabber on the iOS Simulator
- Fixes issue with arm64 not compiling


This fixes the following issues:
 - iOS videoGrabber not working on arm64 https://github.com/openframeworks/openFrameworks/issues/3387

- ios videoGrabberExample should build on simulator https://github.com/openframeworks/openFrameworks/issues/1659