Change PartialEvaluator.setGState to an async method

Since this method calls `Dict.get` to fetch data, there could thus be `Error`s thrown in corrupt PDF documents when attempting to resolve an indirect object.
To ensure that this won't ever become a problem, we change the method to be `async` such that a rejected Promise would be returned and general OperatorList parsing won't break.

src/core/evaluator.js

@@ -832,7 +832,7 @@ class PartialEvaluator {
     throw reason;
   }
 
-  setGState({
+  async setGState({
     resources,
     gState,
     operatorList,