Blender Git Loki

Cycles: Use more stable version of integer square root function

Old code was working quite unreliable in combination with fast math
flag, especially when compiling with Clang. It seems we were hitting
result of the following bug submitted to Clang [1].

Basically, it was happening so that (int)sqrtf(64) was 7 when Cycles
is built with Clang but was correct 8 when built with GCC.

This commit works this around. Annoying, but don't see other way to
keep sampling pattern the same for Clang and GCC.

[1] https://bugs.llvm.org//show_bug.cgi?id=24063