Based on CTS testing, math optimizations between MSL and Vulkan are inconsistent.
In some cases, enabling MSL's fast-math compilation option matches Vulkan's math
results. In other cases, disabling it does. Broadly enabling or disabling fast-math
across all shaders results in some CTS test failures either way.
To fix this, selectively enable/disable fast-math optimizations in the MSL code,
using metal::fast and metal::precise function namespaces, where supported, and
the [[clang::optnone]] function attribute otherwise.
Adjust SPIRV-Cross unit test reference shaders to accommodate these changes.
Need to make output 100% exact for MSVC and GCC libc testing, but they are 1 ULP
off when converting fp32 to string in some weird corner cases.
Roundtrip should be correct though, but that outside the scope of
SPIRV-Cross.
