According to https://github.com/Intel-Media-SDK/MediaSDK/blob/master/doc/mediasdk-man.md#configuration-change.
Before calling MFXVideoENCODE_Reset, The application needs to retrieve
any cached frames in the SDK encoder.
A loop is added before MFXVideoENCODE_Reset to retrieve cached frames
and add them to async_fifo, so that dynamic configuration works when
async_depth > 1.
Signed-off-by: Wenbin Chen <wenbin.chen@intel.com>