Fix aom_fdct32x32_avx2 output as CONFIG_AOM_HIGHBITDEPTH=1

- Change FDCT32x32_2D_AVX2 output parameter to tran_low_t.
- Add unit tests for CONFIG_AOM_HIGHBITDEPTH=1.
- Update TODO notes.
BUG=webm:1323

Change-Id: If4766c919a24231fce886de74658b6dd7a011246
7 files changed