Use vsync instead of delay-based synchronization

Also changed the game_update() interface to take dt in seconds as a
double (rather than milliseconds as an unsigned integer) as we're now
computing the refresh interval as a double anyway (by querying the
display mode).  To my eye (and on my machine), all chunkiness is now
gone; the game feels very smooth.

engine/include/engine_hooks.h

@@ -27,7 +27,7 @@ void game_init(int argc, char *argv[], void *mem, SDL_Renderer *renderer);
 void game_teardown(void *mem);
 
 gamestatus_t game_evthandle(void *mem, const SDL_Event *evt);
-gamestatus_t game_update(void *mem, unsigned dt);
+gamestatus_t game_update(void *mem, double dt);
 
 void game_render(const void *mem, SDL_Renderer *renderer, long unsigned t);