Fixed|GCC: Further qualified type of Materials::Instance::groups

GCC does not automatically make the symbols of the outer class/type
visible to types internal to said outer class/type.

doomsday/engine/src/resource/materials.cpp

@@ -121,7 +121,7 @@ struct Materials::Instance
     MaterialList materials;
 
     /// Animation groups.
-    AnimGroups groups;
+    Materials::AnimGroups groups;
 
     /// Total number of URI material bindings (in all schemes).
     uint bindingCount;