Fix warnings reported by -Wshadow: Part1b: scan_order struct and variable

- Change struct name to all caps SCAN_ORDER to be locally consistent.
- Rename struct pointers to 'scan_order' instead of hard to read short
  names 'so' and 'sc'.

Cherry-picked from aomedia/master: 30abc082

Change-Id: Ib9f0eefe28fa97d23d642b77d7dc8e5f8613177d
11 files changed