retain context because it is a copy of an older ctx
retain context because it is a copy of an older ctx

JuliaPackageMirrors/OpenCL.jl,yuyichao/OpenCL.jl,tkelman/OpenCL.jl,yuyichao/OpenCL.jl,tkelman/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl