…elper

This will be used shortly for checking if we should proceed with
enabling GPU-based validation.

If [`VK_LAYER_KHRONOS_validation`] is present, and it supports
[`VK_EXT_validation_features`], we can configure it with another instance
creation info. element of type [`VkValidationFeaturesEXT`] to enable
GPU-based validation. Wire `InstanceFlags::GPU_BASED_VALIDATION` to do
this in the Vulkan backend. It's even already finding issues in our
`examples` and other tests! But…we'd like to handle those later, since
this is so important for users. So, I've broken that out to separate
issues. The instances we're aware of:

* `water` is running into sync. validation issues: see
  <gfx-rs#5231>
* `wgpu_test::shader::struct_layout::uniform_input` is failing to
    instrument shaders now; see
    <gfx-rs#5245>

It is apparent from this and the [DX12 implementation of GPU-based
validation] that we will need to communicate clearly to users that
`InstanceFlags::GPU_BASED_VALIDATION` implies
`InstanceFlags::VALIDATION`. Not all backends enforce this yet; I have
[split out this work][follow-up for flag implication].

Note that `VK_EXT_validation_features` has been deprecated in favor of
the more general layer configuration mechanism offered by
[`VK_EXT_layer_settings`].

[DX12 implementation of GPU-based validation]: gfx-rs#5146
[`VK_EXT_layer_settings`]: https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_EXT_layer_settings.html
[`VK_EXT_validation_features`]: https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_EXT_validation_features.html
[`VK_LAYER_KHRONOS_validation`]:https://vulkan.lunarg.com/doc/sdk/1.3.275.0/linux/khronos_validation_layer.html
[`VkValidationFeaturesEXT`]: https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VkValidationFeaturesEXT.html
[follow-up for flag implication]: gfx-rs#5232