Related to #206 , cc @grorg
Vulkan and Metal disagree on what the "packed" means:

• in Vulkan, packed formats on little-endian systems have reverse order of components
• in Metal, packed formats on little-endian systems have straight order of components
• D3D12 doesn't have "packed" semantics, and given that it's only available on little-endian systems, it's effectively matching Metal

I suggest we don't define the "packed" in any confusing way, and just make the component ordering straight, like in Metal and D3D12.