glsl: Add a lowering pass for packing varyings.

This lowering pass generates GLSL code that manually packs varyings
into vec4 slots, for the benefit of back-ends that don't support
packed varyings natively.

No functional change--the lowering pass is not yet used.

Reviewed-by: Eric Anholt <eric@anholt.net>

v2: Don't use ir_hierarchical_visitor--just loop over instructions
directly.  Also, make the names of the packed varyings include the
names of the original varyings that were packed into them.

src/glsl/ir_optimization.h

@@ -75,6 +75,9 @@ bool lower_quadop_vector(exec_list *instructions, bool dont_lower_swz);
 bool lower_clip_distance(gl_shader *shader);
 void lower_output_reads(exec_list *instructions);
 void lower_ubo_reference(struct gl_shader *shader, exec_list *instructions);
+void lower_packed_varyings(void *mem_ctx, unsigned location_base,
+                           unsigned locations_used, ir_variable_mode mode,
+                           gl_shader *shader);
 bool optimize_redundant_jumps(exec_list *instructions);
 bool optimize_split_arrays(exec_list *instructions, bool linked);