Initial OpenCL arch module, forked from PTX

Inline stub test program runs standalone:

    clang -framework OpenCL -DTEST_STUB architecture.opencl.stdlib.cpp architecture.posix.stdlib.cpp && ./a.out
Merge branch 'master' of github.com:halide/Halide
Old work in progress on OpenCL
Merge branch 'master' of github.com:halide/Halide
Hack reference demosaic for faster OpenMP parallelism

It seems the parallel loop was completely hindered by the operator()
accessors on the output image. Writing output into a local scratch
array significantly improves results.
Merge branch 'master' of github.com:halide/Halide