Adds a speed feature to eliminate full-rd computation if the modeled
rd or rd based on a different parameter in the same mode is already
a lot larger than the best rd yet.

Specifically, only search the sharp and smooth filters if the modeled
rd cost based on the  regular filter is within a certain factor of the
best rd cost so far. Also, skip full-rd computation of non splitmv
inter modes if the modeled rd cost based on pred error is within the
same factor of the best rd cost so far.

Also adds some enhancements in the rd search for splitmv mode to
speed things up by early breakouts. Negligible impact on performance.

Resuts on derfraw300:
psnr:    -0.013% with the splitmv enhancements, -0.24% with the rd
         breakout feature on.
speedup: 6% with splitmv enhancements, 20% with also residual breakout
         (tested on football sequence at 600 Kbps)

Change-Id: I37abc308ea9f110c1679ce649b6a7e73ab1ad5fc