Add png draw example

examples/Advanced/Display/png_spiffs/png_spiffs.ino

@@ -0,0 +1,32 @@
+/* 
+ * Need to format SPIFFS the first time you run a
+   test or else use the SPIFFS plugin to create a partition
+   https://github.com/me-no-dev/arduino-esp32fs-plugin 
+ * put png file in (<project directory>/data)
+ * Select Tools > ESP32 Sketch Data Upload menu item. This should start uploading the files into ESP32 flash file system.
+*/
+#include <M5Stack.h>
+
+void setup()
+{
+  M5.begin();
+  M5.Power.begin();
+  if(!SPIFFS.begin(true)){
+    Serial.println("SPIFFS Mount Failed");
+    return;
+  }
+
+  M5.Lcd.drawPngFile(SPIFFS, "/test.png", 0, 0);
+}
+
+void loop()
+{
+  M5.update();
+  if(M5.BtnA.wasPressed()) {
+    M5.Lcd.drawPngFile(SPIFFS, "/test.png", 0, 0);
+  }
+
+  if(M5.BtnB.wasPressed()) {
+    M5.Lcd.drawPngFile(SPIFFS, "/test1.png", 0, 0);
+  }
+}