eliminate type ambiguity for std::accumulate() invocation

else clang/libc++ cannot decide between bool and a reference into a bit-vector
specialization

(cherry picked from commit 6a88a212)

Change-Id: I694bbad82b1a05ebe86c5c941f3ac85c71f5fc9e

slideshow/source/engine/activities/activitiesfactory.cxx

@@ -557,7 +557,7 @@ public:
         // interpolate between nIndex and nIndex+1 values
         (*mpAnim)(
             getPresentationValue(
-                accumulate( maValues.back(),
+                accumulate<ValueType>( maValues.back(),
                             mbCumulative ? nRepeatCount : 0,
                             maInterpolator( maValues[ nIndex ],
                                             maValues[ nIndex+1 ],
@@ -577,7 +577,7 @@ public:
         // this is discrete, thus no lerp here.
         (*mpAnim)(
             getPresentationValue(
-                accumulate( maValues.back(),
+                accumulate<ValueType>( maValues.back(),
                             mbCumulative ? nRepeatCount : 0,
                             maValues[ nFrame ] ) ) );
     }