Fix uninitialized blk_skip for VAR TX.
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