glsl: Add ir support for `sample` qualifier; adjust compiler and linker

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 21c1bdd..9e8eca5 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -765,6 +765,12 @@
 			    mode_string(var), var->name);
                return;
             }
+            if (existing->sample != var->sample) {
+               linker_error(prog, "declarations for %s `%s` have "
+                            "mismatching sample qualifiers\n",
+                            mode_string(var), var->name);
+               return;
+            }
 	 } else
 	    variables.add_variable(var);
       }