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.
Which means that reduz' beloved style which we all became used to
will now be changed automatically to remove the first empty line.
This makes us lean closer to 1TBS (the one true brace style) instead
of hybridating it with some Allman-inspired spacing.
There's still the case of braces around single-statement blocks that
needs to be addressed (but clang-format can't help with that, but
clang-tidy may if we agree about it).
Part of #33027.
