SimulcastConsumer: Fix frozen video when syncing keyframe is discarde…

…d due to too high RTP timestamp extra offset needed

- It may fix #990.
- Rationale here: #990 (comment).
- Basically we should reset this->syncRequired` nor `this->spatialLayerToSync` when the keyframe that is supposed to switch to another spatial layer is discarded due to too high RTP timestamp extra offset needed.

worker/src/RTC/SimulcastConsumer.cpp

@@ -809,6 +809,10 @@ namespace RTC
 
 					this->keyFrameForTsOffsetRequested = true;
 
+					// Reset flags since we are discarding this key frame.
+					this->syncRequired       = false;
+					this->spatialLayerToSync = -1;
+
 					return;
 				}