This patch turns on the fixed-depth TX code in the Daala toplevel TX

A REGRESSION IS EXPECTED as this is temporarily dropping Daala TX back
to Q3, which is reduced operating precision over current master.

subset-1:
monty-rest-of-stack-RDO-s1@2017-11-13T14:39:17.093Z ->
 monty-rest-of-stack-Q3-s1@2017-11-13T14:39:52.160Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0927 |     N/A | -0.0001 |   0.1390 | 0.0871 |  0.0835 |     0.0826

objective-1-fast --limit=4:
monty-rest-of-stack-RDO-o1f4@2017-11-13T14:38:57.951Z ->
 monty-rest-of-stack-Q3-o1f4@2017-11-13T14:39:32.205Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0981 |  0.2341 |  0.4784 |   0.1215 | 0.0761 |  0.1144 |     0.1444

Change-Id: Ibbe17226dd47980da632814422d6201c9fc6fa36