Re-sent the H264 buffer when format change detected

When format change detected for AVC, apply same policy
as VP8 does, to sent the current stream buffer to decoder
again after flush, this aims to avoid lost of IDR frame
for reference.

Signed-off-by: Li Xiaowei <xiaowei.a.li@intel.com>
Merge pull request #1 from xiaowei0916/staging

Re-sent the H264 buffer when format change detected