d3d12: skip CB register 0 when shader has no uniforms
UBO binding 0 corresponds to CBV register 0. This binding is reserved to lowering shader uniforms. If the shader does not have uniforms, the slot goes unused. The d3d12_create_root_signature logic needs to set the base register to 1 if there are no uniforms using register 0.
Edited by Bill Kristiansen