Masked joint spatio-temporal prediction

Exploit wedge partition in joint spatio-temporal prediction. One
slice will be intra predicted. The other slice will be inter
predicted.

Bit-rate reduction:
+0.583% derf        (+0.307 on top of interintra)
+1.298% stdhdraw250 (+0.367% on top of interintra)

Change-Id: Iec4bba5a47d0419778458c25b550574a42b3a250
New flags for masked compound inter-inter/inter-intra

Masked inter-inter will be enabled when CONFIG_MASKED_INTERINTER is
on. Masked inter-intra will be enabled only when both
CONFIG_MASKED_INTERINTRA and CONFIG_INTERINTRA are on.

Change-Id: I57efcfe6a3ef2d53129ef703030366503dfa3762
Make Rand8Extremes more extreme

Previous code didn't do what was expected, we want numbers within 16 of
the extrema.

Change-Id: I20c18627c482ec66e8405ddad74ca9276c0c65dc
Fixed a bug in recursive extrapolation filter for intra prediction

Estimation of local mean, which is used to get zero-mean signals
before linear filtering, is corrected.

Change-Id: If73d0ae479201fc60a34baa3f15d61e5aecb1162