Draft: Release 1.8.1.

Release for 1.8.1 containing targeted fixes for issues uncovered by Wine tests, in particular an assert which is currently preventing Wine tests from running with 1.8.

There were other "safe fix" patches I noticed after 1.8, but on the precedent of 1.7.1 I didn't pick any of them.

I've marked this as a draft since it's supposed to be a merge request into a vkd3d-1.8.x branch, but that doesn't currently exist, and it'll need Alexandre to create it as I understand. In the meantime I'm submitting the patches so they can be reviewed.