They're now disabled by default, and can be enabled with the command line
argument `--vk-layers`.
When enabled, the errors about them being missing are now warnings, as
users were confused and thought this meant Vulkan is broken for them.
Fix crash in `~VulkanContext` when validation layers are disabled (exposed by
this PR since before they could not be disabled without source modification).
Also moved VulkanContext member initializations to header.
Fixes#37102.
Rémi Verschelde
PouleyKetchoupp