Disable this feature unless the entire block is within the frame.
The reason is, rd decisions in mbmi, e.g. inter_tx_block[][], made
for blocks partially out of the border can be partly nonsense
therefore cannot be reused by blocks at other locations.

It caused an infinite loop when encoding a clip with repetitive
patterns. A cross-border block has an invalid big tx stored
in inter_tx_block[0][1] and the other block (same residue, within
frame) reused this mbmi, which makes encoder never reach the
termination condition when tx blocks are being recursively
partitioned.

BUG=aomedia:913

Change-Id: Id25a1dbc4a68b5136f6bdf9f6b5811b7ec6920b0