Fix 8X32 iscan table

Add the last scan position in the 8x32 iscan table. This point
would cause the encoder to ignore the last non-zero coefficient
at scan index 255, hence causing enc/dec mismatch. This commit
fixes and solves this issue.

BUG=aomedia:1193

Change-Id: Ia41ba31ef8549f8809cea3d76ecd91da77e130cf