core/umat: usage flags (with proposals from PR #2195)
added [s|l]rgb <-> lab conversion
extended performance tests to test 3 channels
extended performance test
Merge pull request #2340 from ilya-lavrenov:tapi_lab
Merge pull request #2334 from alalek:umat_usage_flags_2
Merge pull request #2345 from ilya-lavrenov:tapi_3cn_perf