Relative to previous ext-inter:
lowres: -0.177%
     or -0.029% (with USE_RECT_INTERINTRA = 0)

* When predicting interintra modes, the previous code did not provide
  the intra predictor with the correct context during rdopt. Add an
  explicit 'ctx' parameter to the relevant functions, to provide this
  context.
  This fixes a nondeterminism bug, which was causing test failures in
  *EncoderThreadTest*

* For rectangular blocks, build_intra_predictors_for_interintra needs
  to overwrite part of the context buffer in order to set up the
  correct context for intra prediction. We now restore the original
  contents afterwards.

* Add a flag to enable/disable rectangular interintra prediction;
  disabling improves encoding speed but reduces BDRATE improvement.

Change-Id: I7458c036c7f94df9ab1ba0c7efa79aeaa7e17118