update hello-opencl

  - break dependency on code from src/compute
  - port away from C APIs to friendlier C++ wrapper APIs
  - add DEPS for OpenCL C++ wrapper headers so we can build on Mac
  - factor out a //third_party/opencl GN target

Change-Id: I9e37c6677cfb779021e66f2bd10f97570c450746
Reviewed-on: https://skia-review.googlesource.com/c/191281
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed