Make last frame source available in current frame encoding

While encoding a frame, its last frame source can be used to give
acurate motion information. This patch prevents last frame to be
overwritten so that it is available during current frame encoding.

The last source is scaled when it is necessary. cpi->Last_Source
points to the last source frame.

Change-Id: I0e1ef5e9e1d2badf9d0c7a1a44a7ed5b24c09425