For #4154 - change TFT driver to use virtual GPIO for backlight enable

src/graphics/TFTDisplay.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <GpioLogic.h>
 #include <OLEDDisplay.h>
 
 /**
@@ -39,6 +40,12 @@ class TFTDisplay : public OLEDDisplay
      */
     void setDetected(uint8_t detected);
 
+    /**
+     * This is normally managed entirely by TFTDisplay, but some rare applications (heltec tracker) might need to replace the
+     * default GPIO behavior with something a bit more complex.
+     */
+    GpioPin *backlightEnable;
+
   protected:
     // the header size of the buffer used, e.g. for the SPI command header
     virtual int getBufferOffset(void) override { return 0; }