Rather than backing out all the LGT-related shifting matrices
throughout the existing TX code, separate out and simplify Daala
inverse TX into a single dedicated entry point.  When DAALA_TX is
enabled, CONFIG_HIGHBITDEPTH is also forced, and all of Daala TX
(lowbd and highbd) uses this single TX dispatch.

This patch is purely non-functional changes.

subset 1:
monty-TXtesting-fwd-s1@2017-11-12T05:25:09.557Z ->
 monty-TXtesting-inv-s1@2017-11-12T05:25:43.878Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

objective-1-fast:
monty-TXtesting-fwd-o1f@2017-11-12T05:25:29.386Z ->
 monty-TXtesting-inv-o1f@2017-11-12T05:25:58.897Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

Change-Id: I790e8d7ac08eb214eb712f5441d6e5f76ebddf17