libvpx

Skip the last frame update for some frame repeats.

Where a frame appears to be a repeat of an earlier
frame or frame buffer,  but the first pass code
does not anticipate this (usually because it is matching
the GF or ARF buffer not the last frame buffer), do not
update the last frame buffer.

This helps ensure that the content of the last frame buffer
is kept "different" where possible, and not updated to
match the GF or ARF. This is particularly helpful in some
animated sequences where there are groups of repeating
frames. Here it has quite a big impact. However, in most
of our standard test clips it has little or no impact.

Change-Id: I77332ee1a69f9ffc0c6080bfeb811c43fd8828e6