add refine mc.cpp on decoder
1. remove unecessary init
2. add static to constant function pointer array
Remove useless ifdefs

This is in practice always enabled on all currently supported
platforms.
Merge pull request #302 from mstorsjo/remove-useless-ifdef

Remove useless ifdefs
Merge pull request #297 from licaiguo/refine-mc.cpp

add refine mc.cpp on decoder