codecs: mpeg2decoder: drain() only when significant sequence changes.
There are a lot of info in the mpeg2's sequence(also including ext display_ext and scalable_ext). We need to notify the subclass about its change, but not all the changes should trigger a drain(), which may change the output picture order. For example, the matrix changes in sequence header does not change the decoder context and so no need to trigger a drain().
Fixes: #899 (closed)