Add Format::UNDEFINED

[Rua]

Changelog:

### Breaking changes
Changes to `Format`:
- The following objects now use `Format` instead of `Option<Format>`: `BufferView`, `Image`, `ImageView`, `SamplerYcbcrConversion`.
- The `block_size` method no longer returns an `Option`.

### Additions
- Added `Format::UNDEFINED`, and implemented `Default` which returns this value.

Following KhronosGroup/Vulkan-Docs#2165, it seems that the Vulkan definition of a "valid VkFormat value" includes UNDEFINED. So this makes that a valid enum value for Vulkano as well, and removes Option APIs where appropriate.

I haven't done it for attachment formats for dynamic rendering. In this case, None doesn't just mean "undefined format", but "no attachment". It matches with the fields of RenderingInfo where the attachments that are absent are also represented as None.

[marc0246]

Looks wonderful, thanks!