add waitvsync() for atari and atari5200

include/atari.h

@@ -227,6 +227,7 @@ extern unsigned char __fastcall__ _getcolor (unsigned char color_reg);
 /* Other screen functions                                                    */
 /*****************************************************************************/
 
+extern void waitvsync (void);                            /* Wait for start of next frame */
 extern int  __fastcall__ _graphics (unsigned char mode); /* mode value same as in BASIC */
 extern void __fastcall__ _scroll (signed char numlines);
                                           /* numlines > 0  scrolls up */

include/atari5200.h

@@ -89,5 +89,8 @@ extern void atr5200std_joy[];        /* referred to by joy_static_stddrv[] */
 */
 #define _bordercolor(color) 0
 
+extern void waitvsync (void);
+/* Wait for start of next frame */
+
 /* End of atari5200.h */
 #endif