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/pattern_helper.js

@@ -14,7 +14,6 @@
  */
 
 import { FormatError, info, Util } from '../shared/util';
-import { WebGLUtils } from './webgl';
 
 var ShadingIRs = {};
 
@@ -145,7 +144,7 @@ var createMeshCanvas = (function createMeshCanvasClosure() {
   }
 
   function createMeshCanvas(bounds, combinesScale, coords, colors, figures,
-                            backgroundColor, cachedCanvases) {
+                            backgroundColor, cachedCanvases, webGLContext) {
     // we will increase scale on some weird factor to let antialiasing take
     // care of "rough" edges
     var EXPECTED_SCALE = 1.1;
@@ -180,10 +179,14 @@ var createMeshCanvas = (function createMeshCanvasClosure() {
     var paddedHeight = height + BORDER_SIZE * 2;
 
     var canvas, tmpCanvas, i, ii;
-    if (WebGLUtils.isEnabled) {
-      canvas = WebGLUtils.drawFigures(width, height, backgroundColor,
-                                      figures, context);
-
+    if (webGLContext.isEnabled) {
+      canvas = webGLContext.drawFigures({
+        width,
+        height,
+        backgroundColor,
+        figures,
+        context,
+      });
       // https://bugzilla.mozilla.org/show_bug.cgi?id=972126
       tmpCanvas = cachedCanvases.getCanvas('mesh', paddedWidth, paddedHeight,
                                            false);
@@ -253,7 +256,7 @@ ShadingIRs.Mesh = {
         // might cause OOM.
         var temporaryPatternCanvas = createMeshCanvas(bounds, scale, coords,
           colors, figures, shadingFill ? null : background,
-          owner.cachedCanvases);
+          owner.cachedCanvases, owner.webGLContext);
 
         if (!shadingFill) {
           ctx.setTransform.apply(ctx, owner.baseTransform);