Add OpenCL accelerated implementation for Retina.
Fix build error when WITH_OPENCL is disabled.
Fix build error on Mac
Let ocl::context only be initialized once in retina_ocl test.
Remove trailing whitespace.
AutoBuffer with initial zero size fixed.
Merge pull request #1309 from pengx17:master_retina_ocl
Merge pull request #1366 from asmorkalov:ocv_autobuffer_fix