Backup and restore vertical scroll offset when goind to sleep/wakeup to avoid scrambled display on wake-up.

src/drivers/St7789.h

@@ -31,6 +31,7 @@ namespace Pinetime {
       private:
         SpiMaster& spi;
         uint8_t pinDataCommand;
+        uint8_t verticalScrollingStartAddress = 0;
 
         void HardwareReset();
         void SoftwareReset();