Split the existing WebGLUtils in two classes, a private WebGLUtils and a public WebGLContext, and utilize the latter in the API to allow various code to access the methods of WebGLUtils

This patch is one (small) step on the way to reduce the general dependency on a global `PDFJS` object, for PDF.js version `2.0`.

src/display/webgl.js

@@ -14,9 +14,35 @@
  */
 /* eslint-disable no-multi-str */
 
-import { getDefaultSetting } from './dom_utils';
 import { shadow } from '../shared/util';
 
+class WebGLContext {
+  constructor({ enable = false, }) {
+    this._enabled = enable === true;
+  }
+
+  get isEnabled() {
+    let enabled = this._enabled;
+    if (enabled) {
+      enabled = WebGLUtils.tryInitGL();
+    }
+    return shadow(this, 'isEnabled', enabled);
+  }
+
+  composeSMask({ layer, mask, properties, }) {
+    return WebGLUtils.composeSMask(layer, mask, properties);
+  }
+
+  drawFigures({ width, height, backgroundColor, figures, context, }) {
+    return WebGLUtils.drawFigures(width, height, backgroundColor, figures,
+                                  context);
+  }
+
+  clear() {
+    WebGLUtils.cleanup();
+  }
+}
+
 var WebGLUtils = (function WebGLUtilsClosure() {
   function loadShader(gl, code, shaderType) {
     var shader = gl.createShader(shaderType);
@@ -405,37 +431,34 @@ var WebGLUtils = (function WebGLUtilsClosure() {
     return canvas;
   }
 
-  function cleanup() {
-    if (smaskCache && smaskCache.canvas) {
-      smaskCache.canvas.width = 0;
-      smaskCache.canvas.height = 0;
-    }
-    if (figuresCache && figuresCache.canvas) {
-      figuresCache.canvas.width = 0;
-      figuresCache.canvas.height = 0;
-    }
-    smaskCache = null;
-    figuresCache = null;
-  }
-
   return {
-    get isEnabled() {
-      if (getDefaultSetting('disableWebGL')) {
-        return false;
-      }
-      var enabled = false;
+    tryInitGL() {
       try {
         generateGL();
-        enabled = !!currentGL;
-      } catch (e) { }
-      return shadow(this, 'isEnabled', enabled);
+        return !!currentGL;
+      } catch (ex) { }
+      return false;
     },
+
     composeSMask,
+
     drawFigures,
-    clear: cleanup,
+
+    cleanup() {
+      if (smaskCache && smaskCache.canvas) {
+        smaskCache.canvas.width = 0;
+        smaskCache.canvas.height = 0;
+      }
+      if (figuresCache && figuresCache.canvas) {
+        figuresCache.canvas.width = 0;
+        figuresCache.canvas.height = 0;
+      }
+      smaskCache = null;
+      figuresCache = null;
+    },
   };
 })();
 
 export {
-  WebGLUtils,
+  WebGLContext,
 };