Fixes infinite loops when call endProcess by adding a flag to check if the filter has already started the end process
GPUImagePicture: check that the memory layout is valid for using the CGImage directly in GL

This corrects the fault introduced by my previous commit enabling CGImages to be used directly in GL.
Merge pull request #1298 from karlvr/gpuimagepicture-redraw2

GPUImagePicture: check that the memory layout is valid
Merge pull request #1295 from achambers16/fix-endprocess-loop

Fix infinite loops related to endProcess on GPUImageFilter