Make the example really need to use const char*

Make the generated GIF dynamic in the example and move the original to
PROGMEM (since that's where const arrays like this belong).

Author: earlephilhower

libraries/ESP8266WebServer/examples/HelloServer/HelloServer.ino

@@ -68,15 +68,20 @@ void setup(void) {
   });
 
   server.on("/gif", []() {
-    static const uint8_t gif[] = {
+    static const uint8_t gif[] PROGMEM = {
       0x47, 0x49, 0x46, 0x38, 0x37, 0x61, 0x10, 0x00, 0x10, 0x00, 0x80, 0x01,
       0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0x2c, 0x00, 0x00, 0x00, 0x00,
       0x10, 0x00, 0x10, 0x00, 0x00, 0x02, 0x19, 0x8c, 0x8f, 0xa9, 0xcb, 0x9d,
       0x00, 0x5f, 0x74, 0xb4, 0x56, 0xb0, 0xb0, 0xd2, 0xf2, 0x35, 0x1e, 0x4c,
       0x0c, 0x24, 0x5a, 0xe6, 0x89, 0xa6, 0x4d, 0x01, 0x00, 0x3b
     };
-
-    server.send(200, "image/gif", gif, sizeof(gif));
+    char gif_colored[sizeof(gif)];
+    memcpy_P(gif_colored, gif, sizeof(gif));
+    // Set the background to a random set of colors
+    gif_colored[16] = millis() % 256;
+    gif_colored[17] = millis() % 256;
+    gif_colored[18] = millis() % 256;
+    server.send(200, "image/gif", gif_colored, sizeof(gif_colored));
   });
 
   server.onNotFound(handleNotFound);