[meta] D3D12 portability concerns

[kvark]

(this list is to be edited)

• unnormalized sampler coordinates - [dx12] Support for unnormalized coordinates in samplers gfx-rs/gfx#2883
• primitive restart index - Primitive strip cut values gfx-rs/gfx#1510
• D3D12 command allocator (analogue of VkDescriptorPool) only allows one command list to be recorded at a time. I thought that we could add a flag for that, since most users don't get affected by this limitation. In the end, we implemented a scheme that dynamically juggles the allocators to avoid this problem in [dx12] Dota-related fixes and improvements gfx-rs/gfx#3393.
• sampler heap limitation of 2048. This prevents users to create descriptor pools with more than 2048 samplers - dx12: Descriptor Pool limitations gfx-rs/gfx#2652
• some of the pipeline states (that can be static or dynamic in Vulkan) are strictly static in D3D12. Namely, depth bias and stencil read/write masks. I suggest we have this restriction encoded in the portability spec. Non-dynamic pipeline states in DX12 gfx-rs/gfx#3441
• unable to reset a resource state with VK_IMAGE_LAYOUT_UNDEFINED - Unable to reset D3D12 texture state with Layout::Undefined gfx-rs/gfx#3401
• VK_IMAGE_LAYOUT_GENERAL supports all operations, but D3D12_RESOURCE_STATE_COMMON is read-only - dx12: Resource transition gfx-rs/gfx#2503
• similarly, access flags for VkBuffer do not directly map to D3D12 mutable resource states
• buffer transitions are for whole resource, there is no sub-range transition
• no truly coherent memory - True coherent memory on DX12 gfx-rs/gfx#3432