Reject ext-inter compound modes based on modelled RD.

Reject ext-inter compound modes before doing full rate distortion
evaluation, if the corresponding single reference modes had a lower
modelled RD.

ext-inter speedup up to TBD.

Coding performance: TBD

Change-Id: I358bfb879c5ebe5e7afbf6f540cc784f8de14857
2 files changed