Use fast HEX search in real time mode

In good quality mode motion search, the best matches are normally
found after searching in a large area. In real time mode, to make
encoding fast, a center-biased fast HEX search is used, which
converges quickly most of the time. A 4-point diamond search is
also carried out as the following refining search, which gives more
precise results, and maintains good motion search quality.

At speed 5, the borg test on rtc set showed an overall PSNR loss of
0.936%. The encoding speed gain is 4% - 5%.

Change-Id: I42cd68bb56a09ca1b86293c99d5f7312225ca7ae