Fix overflow bugs in conversion from NV12 VA-surface/D3D11texture2D to OpenCL UMat

* fix cv::imwrite to allow cv::UMat and vectors of it to be written

* cv::imwrite failure handling improvements

* Unit test for regression testing imwrite support for and fix whitespace

CPATH entries are processed before any -isystem:
https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html