This removes coupling between the renderer and UI/client functions. An option USE_RENDERER_DLOPEN was added to specify whether a renderer module should be compiled and loaded, instead of integrating the renderer into the executable directly. This opens the door for a new renderer
The macOS client and server were completely unusable when run from a
terminal. They blocked forever in `[NSApp run];` which was called by
Sys_InitProtocolHandler(). `applicationDidFinishLaunching` was never
called to exit the NSApp run loop.
Use SDL's SDL_DROPFILE event to receive URLs to handle on macOS instead.
This also handles URLs while the game is running (connect to new server)
instead of nothing happening when clicking a link while the game is
running.