This is a proposed API change in 946, which won't make it into this release.
The intent is to allow slightly larger constants to be specified in the updated
vkd3d_shader_parameter1 structure. In particular, this is large enough to pass
4-dimensional integer or float vectors inline, which the proposed clip plane
implementation will use, as well as other Direct3D FFP parameters.
We could also simply add vkd3d_shader_immediate_constant1 as a separate union
member in vkd3d_shader_parameter1, but this API is a bit cleaner and simpler.