zink: Fixed MacOS compiling issues
Added a moltenvk-dir build option to supply the MacOS Vulkan SDK MoltenVK location.
Forced compiler, for zink only, into object-c mode when MoltenVK is used to allows for the MacOS ioSurface and CAMetalLayer types that the headers expose.
Protected the DRI2 galliumdrm_driver_api and galliumdrm_driver_extensions variables from __APPLE__
builds, as DRI2 cannot be compiled.
Fixed issue with moved screen->have_moltenvk value.