anv/pipeline: Add shader lowering for multiview

v2 (Jason Ekstrand):
 - Take a view_mask rather than a whole subpass
 - Build the view mask into the VS shader key

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/vulkan/anv_nir.h

@@ -35,6 +35,8 @@ void anv_nir_lower_input_attachments(nir_shader *shader);
 
 void anv_nir_lower_push_constants(nir_shader *shader);
 
+bool anv_nir_lower_multiview(nir_shader *shader, uint32_t view_mask);
+
 void anv_nir_apply_pipeline_layout(struct anv_pipeline *pipeline,
                                    nir_shader *shader,
                                    struct brw_stage_prog_data *prog_data,