glsl/linker: Check that re-declared, inter-shader built-in blocks match

>From GLSL 4.5 spec, section "7.1 Built-In Language Variables", page 130 of
the PDF states:

    "If multiple shaders using members of a built-in block belonging to
     the same interface are linked together in the same program, they must
     all redeclare the built-in block in the same way, as described in
     section 4.3.9 “Interface Blocks” for interface-block matching, or a
     link-time error will result."

Fixes:
* GL45-CTS.CommonBugs.CommonBug_PerVertexValidation

v2 (Neil Roberts):
Explicitly look for gl_PerVertex in the symbol tables instead of
waiting to find a variable in the interface.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102677
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
Signed-off-by: Neil Roberts <nroberts@igalia.com>

src/compiler/glsl/link_interface_blocks.cpp

@@ -364,6 +364,35 @@ validate_interstage_inout_blocks(struct gl_shader_program *prog,
                                    consumer->Stage != MESA_SHADER_FRAGMENT) ||
                                   consumer->Stage == MESA_SHADER_GEOMETRY;
 
+   /* Check that block re-declarations of gl_PerVertex are compatible
+    * across shaders: From OpenGL Shading Language 4.5, section
+    * "7.1 Built-In Language Variables", page 130 of the PDF:
+    *
+    *    "If multiple shaders using members of a built-in block belonging
+    *     to the same interface are linked together in the same program,
+    *     they must all redeclare the built-in block in the same way, as
+    *     described in section 4.3.9 “Interface Blocks” for interface-block
+    *     matching, or a link-time error will result."
+    *
+    * This is done explicitly outside of iterating the member variable
+    * declarations because it is possible that the variables are not used and
+    * so they would have been optimised out.
+    */
+   const glsl_type *consumer_iface =
+      consumer->symbols->get_interface("gl_PerVertex",
+                                       ir_var_shader_in);
+
+   const glsl_type *producer_iface =
+      producer->symbols->get_interface("gl_PerVertex",
+                                       ir_var_shader_out);
+
+   if (producer_iface && consumer_iface &&
+       interstage_member_mismatch(prog, consumer_iface, producer_iface)) {
+      linker_error(prog, "Incompatible or missing gl_PerVertex re-declaration "
+                   "in consecutive shaders");
+      return;
+   }
+
    /* Add output interfaces from the producer to the symbol table. */
    foreach_in_list(ir_instruction, node, producer->ir) {
       ir_variable *var = node->as_variable();