The offset stored for a frame was not always relative to the previous
frame. This was the case for reverse movement, but for forwrad movement
the offset was the one to the next frame.

Make the offset handling consistent and alwasy store the offset to the
previous frame. Also update the frame assembling code to add the offset
before blitting the frame (i.e. making it relative to the previous frame
there too).

Note that fpi_assemble_lines already made the assumption that this was
the case as it forced the offset for the first frame to be zero. As
such, the code was inconsistent.

This could affect the AES drivers slightly as they use hardware reported
values which might not adhere to these assumptions.