Fix handling intra period not multiple of GOP length
Fix handling intra period not multiple of GOP length

With low delay GOP structure, it is possible to use an intra period that
is not a multiple of the GOP structure length. Commit 00c9f52 changed
encoder_state_init_new_frame to reset POC on intra frames. GOP offset,
however, was not reset, resulting in invalid POCs and references for the
following frames.

This commit changes function kvz_encoder_feed_frame so that GOP offset
is correctly reset on intra frames.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar