Use rectangular transforms for >= 8x8 blocks

For rectangular blocks between 8x8 and 32x32, we can now code the
transform size as one bigger than the largest square that fits in
the block (eg, for 16x8, we can code a transform size of 16x16
rather than the previous maximum of 8x8), when this oversized
transform is coded in the bitstream, the codec will use the full
size rectangular transform for that block (eg 16x8 transform in
the above example).

Also fixes a scaling bug in 16x8/8x16 transforms.

Change-Id: I62ce75f1b01c46fe2fbc727ce4abef695f4fcd43