Fix narrowing conversion in `{}`

[strega-nil-ms]

We've (the compiler team) started enforcing that {} initialization can't do narrowing conversions in more places, which means that this code will be broken in a new version of the compiler; this is one method to fix this; the alternative is to compare with 0 in the initialization of a few structure members down-file.

Note that this does not break ABI, due to the following pointer member.

old:

class D3D12TranslationLayer::BatchedContext::CreationArgs       size(16):
        +---
 0.     | CreatesAndDestroysAreMultithreaded (bitstart=0,nbits=1)
 0.     | SubmitBatchesToWorkerThread (bitstart=1,nbits=1)
        | <alignment member> (size=4)
 8      | pParentContext
        +---

new:

class D3D12TranslationLayer::BatchedContext::CreationArgs       size(16):
        +---
 0.     | CreatesAndDestroysAreMultithreaded (bitstart=0,nbits=1)
 0.     | SubmitBatchesToWorkerThread (bitstart=1,nbits=1)
        | <alignment member> (size=7)
 8      | pParentContext
        +---

It also works in 32-bit ABIs since the pointer member will have align 4 -- instead of a 7-byte/3-byte alignment member, you'll get a 3-byte/0-byte alignment member, but the size stays the same.