fix a strange static initialization problem on macOS
fix a strange static initialization problem on macOS

It looks like using `half` during static initialization causes issues,
where sometimes these values are not initialized correctly.
The problem goes away when some other random static initializer 
executes, which suggests a race, except there is only a single thread
at that point.

I think for some reason `half4` are not put in the data section where
float4 are.

note: we don't understand the exact issue yet
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