vp9-rtc: tx_size selection for intra mode in nonrd

vp9-rtc: tx_size selection for intra mode in nonrd

In nonrd_pickmode for intra modes: add tx_size selection
based on Y prediction signal for the bsize.

The tx selection is done in model_rd, same as inter-modes.

Existing code for intra mode was first setting a tx_size based
only on the bsize, and then in some cases in block_yrd
(during the loop over bsize in units of tx_size) the tx_size
may be set again if model_rd is called in block_yrd.

This CL separates out the tx_size setting (based on Y channel
prediction via model_rd), and then block_yrd is called once
for whole bsize. This allows for better tuning of the tx
selection for intra modes in future change.

Adjust threshold in svc datarate test.

Negligible/neutral change in psnr/ssim metrics
for speed 7 and 8, 1 layer and SVC mode.

Change-Id: I33bc8447afdc3785482e13aac5c3636e13c59644