intrabc: Add initial skeleton

Missing features:
* RDOPT (Forced on for certain blocks)
* Any form of border extension
* Non MI sized right and bottom edges
* MV prediction

Present features:
* Force intrabc for some blocks
* Carry intrabc in the bitstream
* Validate DV is causal
* Reconstruct intrabc block assuming border extension is unnecessary

Change-Id: Ib1f6868e89bfacc2a4edfc876485bad1b347263b
15 files changed