added contrast control function for gLCDs

Marlin/Configuration.h

@@ -517,6 +517,13 @@ const bool Z_ENDSTOPS_INVERTING = true; // set to true to invert the logic of th
   #endif
 #endif
 
+// default LCD contrast for dogm-like LCD displays
+#ifdef DOGLCD
+# ifndef DEFAULT_LCD_CONTRAST
+#  define DEFAULT_LCD_CONTRAST 32
+# endif
+#endif
+
 // Increase the FAN pwm frequency. Removes the PWM noise but increases heating in the FET/Arduino
 //#define FAST_PWM_FAN