Win32 aliases `size_t` to `unsigned int`, causing
`static_cast<size_t>(V)` to be a useless cast when `V` has type
`unsigned int`. Switch to using direct initialization, which is not a
cast and so does not trigger a warning, but does produce the correct
type. This form disallows narrowing, so inputs that might change in
value as a result of the conversion are an error. Since this is a
sanity checking macro, that is a useful safety measure.
Reported-by: AlumiuN <https://github.com/dxx-rebirth/dxx-rebirth/issues/678#issue-1437577368>
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