Revert: frag shader to original state. Not using GPU after all.

vImage update: processing function improvement, now estimate heart rate using correct length of array (previously using frame_numer incorrect), updating type definition, using float across different place. Making header file more readable, add sample vImage function for future calls.

Removing previousFrameAverageGreen, as it is used for garbage value.
Adding step1 thresholding, preparing for vDSP.

Red color averaging function using kCVPixelBufferLock_readOnly for better CPU usage;
Removing unnecessary comment.

Note: simulator still disabled, will add Accelerate framework substitute.