Invalidate empty pages at the start of wallpaper

        void updatePage(EngineWindowPage currentPage, int pageIndx, int numPages,

                float xOffsetStep) {

            // to save creating a runnable, check twice

            long current = System.currentTimeMillis();

            // in case the clock is zero, we start with negative time

            long current = SystemClock.elapsedRealtime() - DEFAULT_UPDATE_SCREENSHOT_DURATION;

            long lapsed = current - currentPage.getLastUpdateTime();

            // Always update the page when the last update time is <= 0

            // This is important especially when the device first boots

            if (lapsed < DEFAULT_UPDATE_SCREENSHOT_DURATION

                    && currentPage.getLastUpdateTime() > 0) {

            if (lapsed < DEFAULT_UPDATE_SCREENSHOT_DURATION) {

                return;

            }

            Surface surface = mSurfaceHolder.getSurface();