Backward ABI compatibility with libjpeg-turbo

This patch moves the mozjpeg-specific parameters into the opaque master struct and implements a set of accessor functions to get/set them.  This restores backward ABI compatibility with libjpeg/libjpeg-turbo.  Additionally, a second patch fixes a minor build issue discovered with Visual C++ while working on the backward ABI compatibility stuff (Visual C++ doesn't support C99.)