ext/theora/: Fix theora granulepos calculation.

ext/theora/: Fix theora granulepos calculation.

Original commit message from CVS:
* ext/theora/theora.c: (plugin_init):
* ext/theora/theoradec.c: (theora_dec_from_granulepos),
(theora_dec_src_query), (theora_dec_chain):
* ext/theora/theoraenc.c: (gst_theora_enc_class_init),
(theora_enc_sink_link), (theora_buffer_from_packet),
(theora_push_packet), (theora_enc_chain):
Fix theora granulepos calculation.
Fix overflow in duration/position calculation.
Bump rank to PRIMARY for theoradec.
Use granulepos of last packet to calculate position.
Set keyframe flag on buffers when needed.