render/vulkan: get only available validation layers

[bl4ckb0ne]

Fixes #3283

/cc @nyorain

[nyorain]

This only addresses the layer availability part of #3283. When people really start using the Vulkan renderer, they probably want to run it without the validation layer and so we should offer an option to not enable it (even though I don't think it should be the default yet). Note that currently we consider the debug parameter passed to vulkan_instance_create for layer_count, your change ignores it and always forces the layers when they are available.
With the comments addressed, I'd still be in favor of merging this, we can think about explicitly disabling the layers later on.

[emersion]

When the Vulkan renderer is deemed production-ready, we can remove all of this code: people can always enable it again with VK_INSTANCE_LAYERS.

[bl4ckb0ne]

We could do two iterations of the validation layers, first one to enable them if present, then another one to enable them on demand. I tried to do a bit of both here but it's not very good. I'll rework the patch with your feedback @nyorain and I'll open a new issue for WLR_VK_INSTANCE_LAYERS to do later.

[emersion]

wlroots has migrated to gitlab.freedesktop.org. This pull request has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/3318