Add a gulp image_decoders command to allow packaging/distributing the image decoders (i.e. jpg.js, jpx.js, jbig2.js) separately from the main PDF.js library

Please note that the standalone `pdf.image_decoders.js` file will be including the complete `src/shared/util.js` file, despite only using parts of it.[1] This was done *purposely*, to not negatively impact the readability/maintainability of the core PDF.js code.

Furthermore, to ensure that the compatibility is the same in the regular PDF.js library *and* in the the standalone image decoders, `src/shared/compatibility.js` was included as well.

To (hopefully) prevent future complaints about the size of the built `pdf.image_decoders.js` file, a few existing async-related polyfills are being skipped (since all of the image decoders are completely synchronous).
Obviously this required adding a couple of pre-processor statements, but given that these are all limited to "compatibility" code, I think this might be OK!?

---
[1] However, please note that previous commits moved `PageViewport` and `MessageHandler` out of `src/shared/util.js` which reduced its size.

src/pdf.image_decoders.js

@@ -0,0 +1,45 @@
+/* Copyright 2018 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/* eslint-disable no-unused-vars */
+
+import { getVerbosityLevel, setVerbosityLevel } from './shared/util';
+import { Jbig2mage } from './core/jbig2';
+import { JpegImage } from './core/jpg';
+import { JpxImage } from './core/jpx';
+
+// To ensure that the standalone PDF.js image decoders have the same
+// browser/environment compatibility as the regular PDF.js library,
+// the standard set of polyfills are thus included in this build as well.
+//
+// Given that the (current) image decoders don't use all of the features
+// of the complete PDF.js library, e.g. they are completely synchronous,
+// some of the larger polyfills are thus unnecessary.
+//
+// In an attempt to reduce the size of the standalone PDF.js image decoders,
+// the following polyfills are currently being excluded:
+//  - ReadableStream
+//  - Promise
+//  - URL
+
+const pdfjsVersion = PDFJSDev.eval('BUNDLE_VERSION');
+const pdfjsBuild = PDFJSDev.eval('BUNDLE_BUILD');
+
+export {
+  Jbig2mage,
+  JpegImage,
+  JpxImage,
+  getVerbosityLevel,
+  setVerbosityLevel,
+};

src/shared/compatibility.js

@@ -138,6 +138,11 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 
 // Support: IE, Safari<8, Chrome<32
 (function checkPromise() {
+  if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
+    // The current image decoders are synchronous, hence `Promise` shouldn't
+    // need to be polyfilled for the IMAGE_DECODERS build target.
+    return;
+  }
   if (globalScope.Promise) {
     return;
   }
@@ -157,6 +162,11 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/ */
 (function checkURLConstructor() {
+  if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
+    // The current image decoders doesn't utilize the `URL` constructor, hence
+    // it shouldn't need to be polyfilled for the IMAGE_DECODERS build target.
+    return;
+  }
   // feature detect for URL constructor
   var hasWorkingUrl = false;
   try {

src/shared/streams_polyfill.js

@@ -31,6 +31,17 @@ if (typeof ReadableStream !== 'undefined') {
 if (isReadableStreamSupported) {
   exports.ReadableStream = ReadableStream;
 } else {
-  exports.ReadableStream =
-    require('../../external/streams/streams-lib').ReadableStream;
+  if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
+    class DummyReadableStream {
+      constructor() {
+        throw new Error('The current image decoders are synchronous, ' +
+                        'hence `ReadableStream` shouldn\'t need to be ' +
+                        'polyfilled for the IMAGE_DECODERS build target.');
+      }
+    }
+    exports.ReadableStream = DummyReadableStream;
+  } else {
+    exports.ReadableStream =
+      require('../../external/streams/streams-lib').ReadableStream;
+  }
 }