This allows clients to create views of a swapchain image with a
different format, just like a regular image created with the
`VK_IMAGE_CREATE_MUTABLE_FORMAT_BIT` set. This extension also supports
the additional features provided by `VK_KHR_maintenance2` and
`VK_KHR_image_format_list` for swapchain images.
We... admittedly don't do much with any of this. There wasn't really
anything in Metal stopping clients from doing this to begin with. This
change does, however, set the `MUTABLE_FORMAT` and `EXTENDED_USAGE`
image bits on any swapchain created with the new
`VK_SWAPCHAIN_CREATE_MUTABLE_FORMAT_BIT_KHR`.
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