Simulator: improve simulation of transflective screens

This patch simulates the three possible states of a transflective
LCD: backlight on (bright screen as usual), backlight off (dimmed
screen) and LCD off (black screen).
Makes use of already defined 'BACKLIGHT_OFF_ALPHA'.

Change-Id: I9b0fc79b8d50c29e024ba1e6d9c2501119a7e0e0

firmware/target/hosted/sdl/lcd-bitmap.c

@@ -162,7 +162,15 @@ void sim_backlight(int value)
 #endif
     }
 #else /* LCD_DEPTH > 8 */
+#ifdef HAVE_TRANSFLECTIVE_LCD
+    if (!lcd_active())
+        SDL_SetAlpha(lcd_surface, SDL_SRCALPHA, 0);
+    else
+        SDL_SetAlpha(lcd_surface, SDL_SRCALPHA,
+                        MAX(BACKLIGHT_OFF_ALPHA, (value * 255) / 100));
+#else
     SDL_SetAlpha(lcd_surface, SDL_SRCALPHA, (value * 255) / 100);
+#endif
 #endif /* LCD_DEPTH */
 
     sdl_gui_update(lcd_surface, 0, 0, SIM_LCD_WIDTH, SIM_LCD_HEIGHT,

uisimulator/common/lcd-common.c

@@ -78,6 +78,9 @@ void lcd_remote_set_invert_display(bool invert)
 void lcd_sleep(void)
 {
     lcd_sleeping = true;
+#ifdef HAVE_TRANSFLECTIVE_LCD
+    sim_backlight(0);   /* completely blacken the screen */
+#endif
 }
 
 void lcd_awake(void)