Update frameRate (#216)

* reference: Rename frameRate

Signed-off-by: Ce Gao <ce.gao@outlook.com>

* RLang: Add frame rate var

Signed-off-by: Ce Gao <ce.gao@outlook.com>

Author: gaocegege

examples/reference/frameRateVar/.property.yml

@@ -0,0 +1,8 @@
+category: Environment
+subcategory: 
+description: "
+The system variable <b>frameRateVar</b> contains the approximate frame rate of a running sketch. The initial value is 10 fps and is updated with each frame. The value is averaged over several frames, and so will only be accurate after the draw function has run 5-10 times.
+"
+syntax: ""
+related:
+  - frameRate

examples/reference/frameRate_var/.property.yml

@@ -329,7 +329,7 @@ public void setup() {
   @Override
   public void handleDraw() {
     super.handleDraw();
-    this.renjinEngine.put("frameCount", this.frameCount);
+    this.wrapFrameVariables();
   }
 
   /**
@@ -368,6 +368,11 @@ private boolean isMixMode() {
     return hasSize;
   }
 
+  protected void wrapFrameVariables() {
+    this.renjinEngine.put("frameRateVar", frameRate);
+    this.renjinEngine.put("frameCount", frameCount);
+  }
+
   /**
    * Set Environment variables in R top context.
    */