LibVideo/Matroska: Stop skipping a cue when seeking forward

This would cause seeks where the approximate starting cue index was one
cue before the optimal cue to not change the sample iterator position.
Fixing this issue improves accurate seek speeds significantly.

Userland/Libraries/LibVideo/Containers/Matroska/Reader.cpp

@@ -798,12 +798,11 @@ DecoderErrorOr<void> Reader::seek_to_cue_for_timestamp(SampleIterator& iterator,
     }
 
     while (index < cue_points.size()) {
-        auto const& cue_point = cue_points[index++];
+        auto const& cue_point = cue_points[++index];
         dbgln_if(MATROSKA_DEBUG, "Checking future cue point {}ms", cue_point.timestamp().to_milliseconds());
         if (cue_point.timestamp() > timestamp)
             break;
         prev_cue_point = &cue_point;
-        index++;
     }
 
     TRY(iterator.seek_to_cue_point(*prev_cue_point));