Describe the pull request
This PR introduces support for push constants (or root constants in DX 12). The concept of push constants originates from the Vulkan spec and describes a small chunk of memory that can be directly written to the command buffer and read by shaders on a per-draw-call base. There are certain restrictions, though:

• Push constant memory may not exceed 128 bytes (more can be supported, but is not guaranteed)
• Only one push constant range can be bound to a shader stage. However, multiple shader stages can access the same range.
• Multiple ranges may overlap in different shader stages.

In DirectX, push constants are most closely resembled by root constants, which are directly written into the command buffer as 4 byte chunks of raw memory. Overlapping is not supported in this case, thus the implementation will duplicate overlapping memory blocks.

DXC has a the attribute [[vk::push_constant]], which (in combination with [[vk::offset]]) can be used to directly read push constants in Vulkan. For DirectX, root constants are bound to a register/space, as typical constant buffers.

Related issues

• Closes Add support for push constants. #11