Fix GPU pathtracing under Windows

[mworchel]

As discussed in PR #103, GPU pathtracing did not properly work under Windows because of CUDA crashes. It seems like calling the pow function with argument types (double, float) in CUDA code triggers a crash. While the exact cause is still unknown, a fix is to explicitly make sure, the overload pow(double, double) is called by passing the right argument types.

The PR not only fixes the forward pass but also the backward pass, as seen in this super fancy pose_estimation sample:

[mworchel]

Tested and working on two machines with different GPUs. Just for reference, my CUDA version is 10.1.

EDIT: Tested on a third machine.

[BachiLi]

Thanks a lot!! I'll suggest using Real(1) instead of 1.0 so that we can switch to float if we want.

[mworchel]

Fixed by 77f844e.

[BachiLi]

Thanks!