optimize separable filter by
Added "sep_filter_singlepass" kernel that performs separable filtering in one kernel call
Added appropriate host part - sepFilter2D_SinglePass function and SingleStepSeparableFilterEngine_GPU class
Changed function declarations to enable their usage
update doc to be consisted with headers
intel device guard was added because of perf degradation on some non intel platform.
Merge pull request #2009 from krodyush:pullreq/2.4-opt-131211-sepFilterSinglePass_final