When constructing the intra predictor for rectangular interintra blocks,
the last row/column of the first square is copied back into the source
image (which is the current reconstructed image buffer) before
predicting the second square. The code used to use the height instead
of width for vertical rectangles, and vice versa for horizontal
rectangles, leading to overwriting the block on the right/below. This
leads to an encode/decode mismatch if the right/below block is in a
different tile and is encoded before the current block, which did happen
with multi-threaded encoding tests. This is now fixed.

Change-Id: I073a2a447a98b842b1394d72cc774a78cb296921