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.

game/main.c

@@ -576,7 +576,7 @@ gamestatus_t game_evthandle(void *mem, const SDL_Event *evt)
 	return GAMESTATUS_OK;
 }
 
-gamestatus_t game_update(void *mem, unsigned dt)
+gamestatus_t game_update(void *mem, double dt)
 {
 	gamestate_t *state = (gamestate_t *)mem;
 
@@ -590,7 +590,7 @@ gamestatus_t game_update(void *mem, unsigned dt)
 		state->p.dir.x /= dmag;
 		state->p.dir.y /= dmag;
 		state->p.tex = state->pwalk;
-		state->p.speed = WALKSPEED / 1000.0;
+		state->p.speed = WALKSPEED;
 	} else {
 		state->p.tex = state->pidle;
 		state->p.speed = 0;