Add nextFrameDEV() function for checking CPU load

keywords.txt

@@ -57,6 +57,7 @@ invert	KEYWORD2
 justPressed	KEYWORD2
 justReleased	KEYWORD2
 nextFrame	KEYWORD2
+nextFrameDEV	KEYWORD2
 notPressed	KEYWORD2
 off	KEYWORD2
 on	KEYWORD2

src/Arduboy2.cpp

@@ -169,6 +169,18 @@ bool Arduboy2Base::nextFrame()
   return post_render;
 }
 
+bool Arduboy2Base::nextFrameDEV() {
+  bool ret = nextFrame();
+
+  if (ret) {
+    if (lastFrameDurationMs > eachFrameMillis)
+      TXLED1;
+    else
+      TXLED0;
+  }
+  return ret;
+}
+
 int Arduboy2Base::cpuLoad()
 {
   return lastFrameDurationMs*100 / eachFrameMillis;

src/Arduboy2.h

@@ -613,10 +613,37 @@ class Arduboy2Base : public Arduboy2Core
    * }
    * \endcode
    *
-   * \see setFrameRate()
+   * \see setFrameRate() nextFrameDEV()
    */
   bool nextFrame();
 
+  /** \brief
+   * Indicate that it's time to render the next frame, and visually indicate
+   * if the code is running slower than the desired frame rate.
+   * **FOR USE DURING DEVELOPMENT**
+   *
+   * \return `true` if it's time for the next frame.
+   *
+   * \details
+   * This function is intended to be used in place of `nextFrame()` during the
+   * development of a sketch. It does the same thing as `nextFrame()` but
+   * additionally will light the yellow TX LED (at the bottom, to the left
+   * of the USB connector) whenever a frame takes longer to generate than the
+   * time allotted per frame, as determined by the `setFrameRate()` function.
+   *
+   * Therefore, whenever the TX LED comes on (while not communicating over
+   * USB), it indicates that the sketch is running slower than the desired
+   * rate set by `setFrameRate()`. In this case the developer may wish to set
+   * a slower frame rate, or reduce or optimize the code for such frames.
+   *
+   * \note
+   * Once a sketch is ready for release, it would be expected that
+   * `nextFrameDEV()` calls be restored to `nextFrame()`.
+   *
+   * \see nextFrame() cpuLoad() setFrameRate()
+   */
+  bool nextFrameDEV();
+
   /** \brief
    * Indicate if the specified number of frames has elapsed.
    *