COMMON: Added kFeatureHiDPI and return it from the backend

backends/graphics/openglsdl/openglsdl-graphics.cpp

@@ -255,6 +255,8 @@ bool OpenGLSdlGraphicsManager::getFeatureState(OSystem::Feature f) const {
 			return _wantsFullScreen;
 		}
 #endif
+	case OSystem::kFeatureHiDPI:
+		return getGraphicsModeScale(0) == 2;
 
 	default:
 		return OpenGLGraphicsManager::getFeatureState(f);

common/system.h

@@ -368,6 +368,11 @@ public:
 		 */
 		kFeatureFilteringMode,
 
+		/**
+		 * Indicates that GUI runs in HiDPI mode
+		 */
+		kFeatureHiDPI,
+
 		/**
 		 * Indicate if stretch modes are supported by the backend.
 		 */