Describe the pull request

This PR implements support for dynamic resource and sampler heaps. Dynamic means, that the descriptor set does not need to know the descriptor type in advance, but rather the type is decided upon binding a descriptor. Descriptors can be directly accessed using the shader model 6.6 ResourceDescriptorHeap and SamplerDescriptorHeap syntax.

Fundamentally, this works by creating a "proxy" descriptor set, that can accept resources of any type for binding by calling IDescriptorSet::bindToHeap. The method then returns the global heap index to the first bound descriptor, which can be passed to a shader in an arbitrary form, which then uses it to index into the heaps. Subsequent descriptors (if any) are indexed sequentially.

To use this feature, the device must be initialized with the GraphicsDeviceFeature::DynamicDescriptors feature. A working sample and tests are available for a more in-depth demonstration.

In the Vulkan backend, this is implemented using VK_EXT_mutable_descriptor_type. This is generally considered less efficient compared to statically specifying the descriptor type, so prefer the traditional approach if possible. However, using this feature might be more efficient if it reduces the number of pipeline state switches.

Related issues

• Resolves Add support for dynamic descriptor types and direct heap access. #164