Fixed embree compilation errors due to the rework of ray masking.

[scschaefer]

Hi,

as @jerumble had mentioned in his comment in the thread of my last pull request, I accidentally broke the compilation if the Embree backend was enabled. These commits fix this issue by adding the required methods and functions to enable users to use geometry masking in Embree once again. I furthermore updated the Google Test framework to the latest version (v.1.8.1) as the old version was generating a lot of warnings.

I'd love to hear what you think, and if you have any feedback or would like something changed, please let me know.

Cheers, Sebastian

[scschaefer]

Would someone be able to post the log of the build checker? The build works fine for me on Windows with VS2017. I'll try out building the code on my Linux VM, later on, to see if I missed some VS specific code/functions.

[scschaefer]

Okay, I just got around to building the branch with my changes on my Linux VM (Ubuntu 18.04.1 LTS with g++ 7.3.0) and the build worked fine. Thus, I would appreciate any info that can help me find the cause for the continuous-integration test failing.