HLSL dynamic resources ("ultimate bindless") support

[dzhdanNV]

D3D12 already supports it just by adding D3D12_ROOT_SIGNATURE_FLAG_SAMPLER_HEAP_DIRECTLY_INDEXED | D3D12_ROOT_SIGNATURE_FLAG_SAMPLER_HEAP_DIRECTLY_INDEXED flags to the root signature. But it needs to be reviewed because the end user works with descriptor heap (pool) through descriptor sets and mapping is unknown (but it can be guessed, because it's trivial). VK support is less trivial because it requires hooking up a descriptor buffer entity. And VK implementation assumes that most likely the entire "ultimate bindless" functionality needs to be organized as a stand alone extension.

VK:

• https://www.khronos.org/blog/vk-ext-descriptor-buffer
• https://registry.khronos.org/vulkan/specs/latest/man/html/VK_EXT_descriptor_buffer.html

D3D12:

• https://microsoft.github.io/DirectX-Specs/d3d/HLSL_SM_6_6_DynamicResources.html