OpenCL: core support for FP16, more channel orders

OpenCL: core support for FP16, more channel orders

* Support cl_image conversion for CL_HALF_FLOAT (float16)

* Support cl_image conversion for additional channel orders:
  CL_A, CL_INTENSITY, CL_LUMINANCE, CL_RG, CL_RA

* Comment on why cl_image conversion is unsupported for CL_RGB

* Predict optimal vector width for float16

* ocl::kernelToStr: support float16

* ocl::Device::halfFPConfig: drop artificial requirement for OpenCL
  version >= 1.2. Even OpenCL 1.0 supports the underlying config
  property, CL_DEVICE_HALF_FP_CONFIG.

* dumpOpenCLInformation: provide info on OpenCL half-float support
  and preferred half-float vector width

* randu: support default range [-1.0, 1.0] for float16

* TestBase::warmup: support float16