vulkan: Add format fallback

vulkan: Add format fallback

When a GdkMemoryFormat isn't supported, pick close formats that have a
higher chance of being supported.
Make sure this works recursively and the whole loop always ends up at
R8G8B8A8_UNORM because that one is mandatory.

Roughly, follow these rules:
1. Drop the unpremultiplied
2. Expand channels to include all of RGBA
3. pick swizzle that is RGBA
4. pick next largest depth
5. pick R8G8B8A8_UNORM