vpx[dec|enc]: Extract IVF support from the apps.

- Move IVF reading support into ivfdec.c and ivfdec.h
- Move IVF writing support into ivfenc.c and ivfenc.h
- Removed IVF writing code from the SVC example in favor of ivfenc.

Change-Id: I70adf6240d0320fdd232d8546ed573f0f68dd793