OpenCL odd dimensions performance fix
OpenCL odd dimensions performance fix

With certain task dimensions OpenCL performans could drop dramatically
on some platforms. Task dimension is now deliberatly increased to the
nearest multiple of 256*CUcount. Benchmarks below.

Execution time, s
Dimension, rays 100000  100003
no fix with fix no fix with fix
Intel Core I7 6700 0.015 0.016 0.046 0.016
Intel Xeon W3565 0.45 0.49 0.121 0.05
Nvidia Geforce GTX 745 0.058 0.058 2.404 0.059
AMD Radeon R9 280X 0.014 0.015 0.015 0.015

kklmn/xrt,kklmn/xrt,kklmn/xrt,kklmn/xrt