OCL: read 16 uchar values in a moment instead of 4

**WIP**
**Optimization:** as in most of cases we work with a full tile (32 uchar values), reading by 16 uchar vector applied

**Performance report:**
http://ocl.itseez.com/intel/export/perf/pr/2916/report/

check_regression=*OCL*Moments*
build_examples=OFF
test_modules=imgproc