Calculate Newtonian gravity using fast Fourier transforms

About 16% slower for one cell changing, same speed for 6 cells changing,
and several hundred times faster for whole screen changing.

includes/air.h

@@ -41,6 +41,11 @@ void update_airh(void);
 
 void update_grav(void);
 
+#ifdef GRAVFFT
+void grav_fft_init();
+void grav_fft_cleanup();
+#endif
+
 void update_air(void);
 
 #endif