build: Add -Woverride-init

build: Add -Woverride-init

That's a gcc warning (clang has the equivalent -Winitializer-overrides,
but that one is included in -Wall) that complains about things like:

  VkOffset3D offset = { .x = pt.x, .x = pt.y, .y = 0 };

So you don't have to spend a few hours trying to understand what's going
on before realizing your copy/paste skills are substandard.