Cleanup: zero initialize VColRef before setting members

Ensures new members will be zeroed, the compiler is able to optimize
this into identical compiled output.

source/blender/draw/intern/mesh_extractors/extract_mesh_vbo_vcol.cc

@@ -49,7 +49,7 @@ static blender::Vector<VColRef> get_vcol_refs(const CustomData *cd_vdata,
         continue;
       }
 
-      VColRef ref;
+      VColRef ref = {};
       ref.domain = domain;
       ref.layer = layer;