Takes care of transforming clip/cull system values from the Direct3D
convention of 2 4-component registers, into the SPIR-V/GLSL convention
of 8-element scalar float arrays.
This fixes SPIR-V validation errors in clip-cull-distance.shader_test,
as well as segfaults on Mesa 25.1.1-arch1.2 if those shaders are
executed regardless.
We create indexable temporaries of the appropriate size, and replace
accesses to clip/cull I/O signature elements with accesses to those
temporaries. The existing clip/cull signature elements are then replaced
with new scalar signature element arrays, and we copy the contents of
those I/O signature elements to/from the corresponding temporaries at
the start/end of the vsir program.
It is worth pointing out that the current implementation assumes that
every instance of the control point phase of a hull shader only writes
to the output registers of its control point, given by
vOutputControlPointID, and not to other control points. Shader
compilation will fail if that constraint is violated.
The validator checks that on normalized vsir I/O destination write masks
are always a subset of the element's write mask.
This is currently not always happening for FOG and PSIZE outputs, because
d3dbc input might use the 0xf mask instead, despite these semantics being
scalar.
Note that this problem is hidden when using varying mapping, because in
that case the vsir_program_remap_output_signature() pass, specifically
the remove_unread_output_components() function, fixes the write mask.
