Haihao Xiang authored May 08, 2019 and Víctor Manuel Jáquez Leal committed Jul 07, 2019

The call of MFXVideoENCODE_EncodeFrameAsync may not generate output and
the function returns MFX_ERR_MORE_DATA with NULL sync point, the input
frame is cached in this case, so it is possible that all allocated
frames go into the surfaces_used list after calling
MFXVideoENCODE_EncodeFrameAsync a few times, then the encoder will fail
to get an available surface before releasing used frames

This patch adds a new field of num_extra_frames to GstMsdkEnc and allows
encode element requires extra frames, the default value is 0.

This patch is the preparation for msdkvp9enc element.