For both low delay and high delay scenarios, the reference pair in
skip mode are specified as the closest fwd ref, together with the
closest bwd ref if there is any bwd ref, otherwise with the two
closest fwd refs.

Skip mode by default uses COMPOUND_AVERAGE. When all the reference
frames are on the same side, temporal-distance weighted compound is
considered, and a compound index is signaled to indicate whether
distance-weighted compound or compound-average is usd.

Whether to use distance-weighted compound for skip mode is still
under experimenting, hence a flag is temporarily added:
SKIP_MODE_WITH_JNT_COMP.

Following experimental results are obtained over 30 frames, using the
setup of --disable-ext-partition --disable-ext-partition-types
--disable-txmg --enable-jnt-comp --enable-mfmv --enable-ext-skip:

(1) High Latency:
For Google test sets (lowres/midres), the BDRate coding gain is ~0.2%;
For AWCY, the coding gain is ~0.1%.
(2) Low Latency:
No gain has been observed over Google sets and ~0.1% gain is obtained
only when temporal-distance weighted prediction is used.

Change-Id: I8c433357adebed0126ebfdd5c4d51aa71e64be57