constexpr problems with MSVC

[ProGTX]

I've noticed that the Gaussian blur sample fails to compile with Visual Studio 2015 using ComputeCpp 0.6:

gaussian-blur.cpp(159): error C2131: expression did not evaluate to a constant
gaussian-blur.cpp(159): note: failure was caused by non-constant arguments or reference to a non-constant symbol

This is referring to this line of the GaussianKernel kernel lambda:

constexpr auto offset = 3 * stddev;

Even though stddev is a constexpr variable that was declared outside the kernel lambda, it's not considered constexpr inside the kernel lambda.

I believe this is a Visual Studio bug that's not even been fixed in MSVC 2017 (https://godbolt.org/g/t7n1uW).

Interestingly, it seems that making the constexpr variable static constexpr solves this problem, so maybe this should be the default way of using constexpr variables inside kernels.

[DuncanMcBain]

Hi, I've pushed a commit that should fix it, though we don't have testing for Windows set up on GitHub. Could you confirm that this fix makes the Gaussian blur work on Windows?

[DuncanMcBain]

Fixed in #98