This returns true if a block signals tx_size in the stream and uses it
in the bitstream writing code and the decoder.

Note that we can't quite use it in pack_inter_mode_mvs when
CONFIG_VAR_TX && !CONFIG_RECT_TX but I've switched the code to using
it the rest of the time since rect-tx is adopted and eventually the
other code path should be deleted.

Also use the helper function in tx_size_cost in rdopt.c, where the
test was wrong and caused underestimates of block
costs. (Specifically, the code that subtracts tx_size_cost from
this_rate_tokenonly in rd_pick_intra_sby_mode ended up subtracting
zero for a 4x8 block).

The behaviour of the decoder should be unchanged. The only change in
the encoder's behaviour should be in tx_size_cost where it should now
match the rest of the code.

Change-Id: I97236c9ce444993afe01ac5c6f4a0bb9e5049217