[Cin] OpenCL again

[Andrea paz]

With system ffmpeg the boxblur filter works. With CinGG's ffmpeg
(thirdparty) it fails to initialize the hardware:

$ ./ffmpeg -init_hw_device opencl=gpu -filter_hw_device gpu -i
/home/paz/kumar.mp4 -vf
'hwupload,boxblur_opencl,hwdownload,format=yuv420p'
/home/paz/test-opencl.mp4
Device creation failed: -12.
Failed to set value 'opencl=gpu' for option 'init_hw_device': Cannot
allocate memory
Error parsing global options: Cannot allocate memory