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);
diff --git a/gui/blanktimer.hpp b/gui/blanktimer.hpp
index 2d83c4d..60d5b1b 100644
--- a/gui/blanktimer.hpp
+++ b/gui/blanktimer.hpp
@@ -30,6 +30,7 @@
 		int setTimerThread(void);
 		void resetTimerAndUnblank(void);
 		void setTime(int newtime);
+		bool IsScreenOff();
 
 	private:
 		void setConBlank(int blank);
@@ -47,6 +48,7 @@
 		timespec btimer;
 		unsigned long long sleepTimer;
 		int orig_brightness;
+		bool screenoff;
 };
 
 extern blanktimer blankTimer;
diff --git a/gui/pages.cpp b/gui/pages.cpp
index 11fe9b0..86d4dff 100644
--- a/gui/pages.cpp
+++ b/gui/pages.cpp
@@ -41,8 +41,10 @@
 
 #include "rapidxml.hpp"
 #include "objects.hpp"
+#include "blanktimer.hpp"
 
 extern int gGuiRunning;
+extern blanktimer blankTimer;
 
 std::map<std::string, PageSet*> PageManager::mPageSets;
 PageSet* PageManager::mCurrentSet;
@@ -890,6 +892,9 @@
 
 int PageManager::Update(void)
 {
+    if(blankTimer.IsScreenOff())
+        return 0;
+
     return (mCurrentSet ? mCurrentSet->Update() : -1);
 }