opencl wrong for long kernels

[pkienzle]

On a MacbookPro? (Intel HD 4000 GPU), the 2D barbell model with length pd=0.2 radius pd=0.2 gives the wrong scattering pattern when Npts=300.

It works fine for smaller N or smaller pd.

Check other cards/other platforms.

If it is very common problem, we can fetch the results from the GPU every ½ second or so, and sum them on the CPU.