tests/check/pipelines/theoraenc.c: It would be very bad if, after a discont buffer, we thought every single following...

Original commit message from CVS:
* tests/check/pipelines/theoraenc.c: (check_buffer_granulepos),
(GST_START_TEST):
It would be very bad if, after a discont buffer, we thought every
single following buffer was also discont. So, add to the test to
ensure that this isn't the case.

* ext/theora/theoraenc.c: (theora_enc_is_discontinuous):
... it was the case. So fix it.