Ensure that the imageformat Qt plugins are loaded even when no QApplication is created

.travis.yml

@@ -6,8 +6,5 @@ env:
 
 before_install:
     - curl https://download.calibre-ebook.com/travis/sw-linux.tar.xz | tar xJ -C $HOME
-    - $QMAKE -query
-    - ldd $SW/qt/plugins/imageformats/libqjpeg.so
-    - python -c "import os, sys; os.environ['QT_DEBUG_PLUGINS']='1'; from PyQt5.Qt import *; print QLibraryInfo.location(QLibraryInfo.PluginsPath); fmts = map(str, QImageReader.supportedImageFormats()); print fmts; sys.exit('jpg' not in fmts)"
     - python setup.py bootstrap --ephemeral
 script: python setup.py test

src/calibre/utils/img.py

@@ -4,17 +4,29 @@
 
 from __future__ import (unicode_literals, division, absolute_import,
                         print_function)
+from future_builtins import map
 import os, subprocess, errno, shutil, tempfile, sys
 from io import BytesIO
 from threading import Thread
 
-from PyQt5.Qt import QImage, QByteArray, QBuffer, Qt, QImageReader, QColor, QImageWriter, QTransform
+from PyQt5.Qt import QImage, QByteArray, QBuffer, Qt, QImageReader, QColor, QImageWriter, QTransform, QCoreApplication, QProcessEnvironment
 
 from calibre import fit_image, force_unicode
 from calibre.constants import iswindows, plugins, get_version
 from calibre.utils.config_base import tweaks
 from calibre.utils.filenames import atomic_rename
 
+# Ensure that Qt can load the imageformat plugins
+
+if QCoreApplication.instance() is None:
+    # Normally constructing a QCoreApplication takes care of this, but if we
+    # are being used without a QCoreApplication, then we have to do it manually
+    if len(set(b'jpeg gif png'.split(b' ')).intersection(set(map(bytes, QImageReader.supportedImageFormats())))) != 3:
+        qpp = QProcessEnvironment.systemEnvironment().value('QT_PLUGIN_PATH')
+        if qpp:
+            for path in qpp.split(os.pathsep):
+                QCoreApplication.addLibraryPath(path)
+
 # Utilities {{{
 imageops, imageops_err = plugins['imageops']
 if imageops is None: