fix(DASH): Fix PERIOD_FLATTENING_FAILED on fastswitching streams (#6113)

Author: avelad

lib/util/periods.js

@@ -1034,6 +1034,10 @@ shaka.util.PeriodCombiner = class {
           continue;
         }
 
+        if (outputStream.fastSwitching != stream.fastSwitching) {
+          continue;
+        }
+
         if (!best || isBetterMatch(outputStream, best, stream)) {
           best = stream;
         }