Apply framerate limiting delay after serving content boundary.

Putting delay after finishing serving the frame causes image to appear
in stream with a lag. The reason for this appears to be that browser
relies on next frame content boundary in order to determine the end of
image.

That means that in order for browser to show frame right after receiving
it, server needs to send next frame content boundary right after image
itself, and delay to limit framerate should be applied after content
boundary, not before it.

According to https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
sending first frame without content boundary will likely cause its
content to be ignored, which seems to be acceptable price for not
treating first iteration of the loop as special case.

app_httpd.cpp

@@ -251,16 +251,16 @@ static esp_err_t stream_handler(httpd_req_t *req){
                 _jpg_buf = fb->buf;
             }
         }
-        if(res == ESP_OK){
-            res = httpd_resp_send_chunk(req, _STREAM_BOUNDARY, strlen(_STREAM_BOUNDARY));
-        }
         if(res == ESP_OK){
             size_t hlen = snprintf((char *)part_buf, 64, _STREAM_PART, _jpg_buf_len);
             res = httpd_resp_send_chunk(req, (const char *)part_buf, hlen);
         }
         if(res == ESP_OK){
             res = httpd_resp_send_chunk(req, (const char *)_jpg_buf, _jpg_buf_len);
         }
+        if(res == ESP_OK){
+            res = httpd_resp_send_chunk(req, _STREAM_BOUNDARY, strlen(_STREAM_BOUNDARY));
+        }
         if(fb){
             esp_camera_fb_return(fb);
             fb = NULL;