D DLL crashes if not run on the main thread

[Richard (Rikki) Andrew Cattermole]

Assuming it works outside of a DLL, you're probably missing the runtime 
initialization calls.

Mix SimpleDllMain in (or do your own). 
https://github.com/dlang/dmd/blob/9639d72ea0883808feff7aba71d87c5a78fb7f92/druntime/src/core/sys/windows/dll.d#L577