glsl: Use a stable attr sort for VS in / FS out

This is a perpetual bug that hits Windows. In the MSVC CRT, qsort
is unstable, where the glibc qsort is stable. So apps run fine on
Windows IHV drivers, and on Linux Mesa drivers, and only break down
when running on Windows Mesa drivers.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/10922
Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28586>

src/compiler/glsl/gl_nir_link_varyings.c

@@ -50,6 +50,7 @@
 /* Temporary storage for the set of attributes that need locations assigned. */
 struct temp_attr {
    unsigned slots;
+   unsigned original_idx;
    nir_variable *var;
 };
 
@@ -61,7 +62,10 @@ compare_attr(const void *a, const void *b)
    const struct temp_attr *const r = (const struct temp_attr *) b;
 
    /* Reversed because we want a descending order sort below. */
-   return r->slots - l->slots;
+   if (r->slots != l->slots)
+      return r->slots - l->slots;
+
+   return l->original_idx - r->original_idx;
 }
 
 /**
@@ -1238,6 +1242,7 @@ assign_attribute_or_color_locations(void *mem_ctx,
       }
       to_assign[num_attr].slots = slots;
       to_assign[num_attr].var = var;
+      to_assign[num_attr].original_idx = num_attr;
       num_attr++;
    }