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.
diff --git a/gui/blanktimer.cpp b/gui/blanktimer.cpp
index 700a9ad..0b1f008 100644
--- a/gui/blanktimer.cpp
+++ b/gui/blanktimer.cpp
@@ -48,6 +48,11 @@
 	setTime(0);
 	setConBlank(0);
 	orig_brightness = getBrightness();
+	screenoff = false;
+}
+
+bool blanktimer::IsScreenOff() {
+	return screenoff;
 }
 
 void blanktimer::setTime(int newtime) {
@@ -92,6 +97,7 @@
 		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 @@
 			// 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);