glsl: add a NIR based varying linker

With a NIR based linker we get better xfb packing, and we no longer
depend on the GLSL IR optimisations to be able to link shaders
with a large amount of dead input/outputs.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15731>

src/compiler/glsl/gl_nir_linker.h

@@ -25,6 +25,7 @@
 #define GL_NIR_LINKER_H
 
 #include "main/glheader.h"
+#include "main/menums.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -58,6 +59,10 @@ bool gl_nir_link_uniforms(const struct gl_constants *consts,
                           struct gl_shader_program *prog,
                           bool fill_parameters);
 
+bool gl_nir_link_varyings(const struct gl_constants *consts,
+                          const struct gl_extensions *exts,
+                          gl_api api, struct gl_shader_program *prog);
+
 void gl_nir_set_uniform_initializers(const struct gl_constants *consts,
                                      struct gl_shader_program *prog);