Update solvepnp.cpp

Clarified assert message for allowed PnP flags
added OpenCL RGB <-> Luv conversions
added ipp threshold inplace
Merge pull request #2697 from PhilLab:master
Merge pull request #2700 from ilya-lavrenov:tapi_cvtColor
Merge pull request #2710 from ilya-lavrenov:ipp_threshold