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
4d38932cb4