pipeline: initialize HashState flags member

There are a set of "EvalFlags" that get passed to _cogl_pipeline_hash
that can tweak the semantics of what state is evaluated for hashing but
these flags weren't getting passed via the HashState state structure
so it would be undefined if you would get the correct semantics.

cogl/cogl-pipeline.c

@@ -5990,6 +5990,7 @@ _cogl_pipeline_hash (CoglPipeline *pipeline,
 
   state.hash = 0;
   state.layer_differences = layer_differences;
+  state.flags = flags;
 
   /* hash non-sparse state */