[engine] 支持预缓存 Label Canvas

engine/cocos2d/core/renderer/webgl/assemblers/label/index.js

@@ -44,29 +44,46 @@ if(CC_JSB) {
 
 Label._canvasPool = {
     pool: [],
+    used: 0,
+    max: 32,
     get () {
         let data = this.pool.pop();
 
         if (!data) {
+            data = this._create();
+        }
+
+        this.used++;
+        return data;
+    },
+    put (canvas) {
+        this.used--;
+        if (this.pool.length >= this.max) {
+            return;
+        }
+        this.pool.push(canvas);
+    },
+    _create() {
         let canvas = document.createElement("canvas");
         let context = canvas.getContext("2d");
-            data = {
+        
+        const data = {
             canvas: canvas,
             context: context
         }
 
         // default text info
         context.textBaseline = 'alphabetic';  
-        }
-
         return data;
     },
-    put (canvas) {
+    cache(count) {
-        if (this.pool.length >= 32) {
+        const target = Math.min(this.max, count);
-            return;
+        let total = this.used + this.pool.length;
-        }
+        while (total < target) {
-        this.pool.push(canvas);
+            this.pool.push(this._create());
+            total++;
         }
+    },
 };
 
 Assembler.register(cc.Label, {