x->blk_skip used to be uninitialized (leftover from encoding the
previous block), if cm->tx_mode != TX_MODE_SELECT (which is used with
higher --cpu-used or --rt options). This resulted in degraded coding
performance when using cm->tx_mode != TX_MODE_SELECT.

This fixes the VP10/EndToEndTestLarge.EndtoEndPSNRTest/40 unit test.

Also fixed an edge effect where encode_block in encodemb.c used the
formal width of the block (without cropping at the right edge), to
look up blk_skip, while select_tx_block in rdopt.c used the cropped
width to set blk_skip.

Change-Id: I76d0f49ac5ab3ab54203573e0d7fcfcc1c6aa10d