This gives a similar structure to libwebp and fixes a bug where passing
NULL to SharpYuvInit() would unconditionally set optimized function
pointers. SharpYuvInit() is left as an undocumented public function and
SharpYuvGetCPUInfo is kept private to serialize updates to the pointer.
Change-Id: Id72fbf3ba5b396367510e3bcd1ee2e4e11b95b8c
