Use OPENNI2_INCLUDE64 environment variable.
(Set on windows with install from http://structure.io/openni).
Mod: Open header files with `io.open` for Python 2/3 compatibility.
Fix canny segment fault (Bug #3978)

Avoid stack overflow based on std::vector cause
std::vector::resize() segmentation fault.
Merge pull request #3407 from wangyan42164:canny_bug_fix
Doxygen documentation for core module
Merge pull request #3406 from shiquanwang:Fix-bug-#4003_build-python-binding-under-python3
Improve the performance of fast_nlmeans_denoising_opencl.

1. Remove unnecessary barriers.
2. Adjust CTA_SIZE based on the following cases for Intel platform:
   a) OCL_Photo_DenoisingGrayscale.DenoisingGrayscale
   b) OCL_Photo_DenoisingColored.DenoisingColored
fix gpu performance sample compilation
Merge pull request #3413 from wangyan42164:denoising_opencl_improvement
Merge pull request #3415 from jet47:fix-gpu-perf-sample
Merge pull request #3399 from mshabunin:doxygen-core
updated linux installation documentation
Merge remote-tracking branch 'upstream/master' into daniel
Merge pull request #3422 from avdmitry:install_doc_update
Merge pull request #3424 from dsjolie:daniel