Refactor lgt

Change get_lgt in order to integrate a later experiment
lgt_from_pred with lgt. There are two main changes.

The main purpose for this change is to unify get_fwd_lgt and
get_inv_lgt functions into a get_lgt function so the lgt basis
functions can always be selected through the same function in
both forward and inverse transform paths. The structure of those
functions will also be consistent with the get_lgt_from_pred
functions that will be added in the lgt-from-pred experiment.

These changes have no impact on the bitstream.

Change-Id: Ifd3dfc1a9e1a250495830ddbf42c201e80aa913e