Eliminate copying for FLIPADST in fwd transforms.

Eliminate copying for FLIPADST in fwd transforms.

This is a port of 01bb4a318dc0f9069264b7fd5641bc3014f47f32

This commit also fixes a bug where FLIPADST transforms when combined
with a DST (that is FLIPADST_DST and DST_FLIPADST) did not actually did
a flipped transform but a straight ADST instead. This was due to the C
implementation that it fell back on not implementing flipping.  This is
now fixed as well and FLIPADST_DST and DST_FLIPADST does what it is
supposed to do.

Change-Id: I89c67ca1d5e06808a1567c51e7d6bec4998182bd