video: Add coded_frame_size meta type.
The new added GstVideoCodedFrameSizeMeta is used for controlling the coded frame size for each frame. This can give a hint to encoder about the coded size of current frame. Because we can not decide type of the frame(I/P/B) before entering the encoder, all three size info are given. The encoder will try the QP steps given in the meta to generate the coded frame size required. This feature can help user to dynamically change the QP and coded size for each frame, especially in constant QP mode.