Do not render the GUI when the blanktimer turns off the display.

Normally, when the display has been turned off by blanktimer, the GUI actually still renders the GUI elements (eg. progressbar while doing backup/restore/etc.)
This patch will check whenever the display is turned off, and if it is turned off simply dont render. This avoids heating up the device and will save battery.

gui/blanktimer.cpp

@@ -48,6 +48,11 @@ blanktimer::blanktimer(void) {
 	setTime(0);
 	setConBlank(0);
 	orig_brightness = getBrightness();
+	screenoff = false;
+}
+
+bool blanktimer::IsScreenOff() {
+	return screenoff;
 }
 
 void blanktimer::setTime(int newtime) {
@@ -92,6 +97,7 @@ int  blanktimer::setClockTimer(void) {
 		if (sleepTimer && diff.tv_sec > sleepTimer && conblank < 2) {
 			setConBlank(2);
 			setBrightness(0);
+			screenoff = true;
 			PageManager::ChangeOverlay("lock");
 		}
 #ifndef TW_NO_SCREEN_BLANK
@@ -146,6 +152,7 @@ void blanktimer::resetTimerAndUnblank(void) {
 			// No break here, we want to keep going
 		case 2:
 			gui_forceRender();
+			screenoff = false;
 			// No break here, we want to keep going
 		case 1:
 			setBrightness(orig_brightness);