Fix compiler warnings for VS2010 /W4 in sao.c.

- Working towards issue #11.
Fix compiler warnings for VS2010 /W4 in context.c and rdo.c.
- Working towards issue #11.
- Moved all const arrays from .h to the .c. These are not used anywhere else.
- Moved entropy_bits array and its helper macro to rdo.c. They are not used
  anywhere else.
- Implicit conversions to explicit ones.