In CWelsDecoder::ReorderPicturesInDisplay(), iLastWrittenPOC was
not set in unbuffered-reordering case. Due to this problem, it
sometimes reordered the frames incorrectly. This patch fixes the
issue.

When bNewSeqBegin got true, iRefCount was forcibly reset even if the
buffer was still used for reordering. Due to this problem, the buffer
in use was sometimes overwritten with newly decoded frame. This commit
is for fixing that problem.

Previously, when more than 2 frame sequences exist in the reordering
buffer, the reordering does not work correctly. This is because the
management based on bLastGOP flag does not work in this case.
Instead, this patch introduces frame sequence number (iSeqNum) to
distingish frames belonging to each sequence and drops bLastGOP flag.

... and fix trivial issue of that function. This solves the some
frame decode error.