Restore backward ABI compatibility with libjpeg/libjpeg-turbo by moving the mozjpeg-specific parameters into the opaque jpeg_comp_master struct and implementing generic accessor functions for getting/setting those parameters. These functions can be used upstream, if the need for them arises in libjpeg-turbo, and they can also be easily extended to cover future extensions to the decompressor. Note that, in order to use jpeg_comp_master as a repository for extension parameters, cinfo->master is now allocated within the body of jpeg_CreateCompress(). It is later re-allocated in jinit_c_master_control(), because that function (and others in jcmaster.c) use an extended form of jpeg_comp_master, but the existing extension parameters are copied into the new master instance. Similar modifications would need to be made to the decompressor to support the same type of extension framework.
Fix build error encountered when using MS Visual C++ (it doesn't support C99) -- at least not the version of MSVC++ I'm using.)