Always require QtOpenGL

It makes it a bit easier to explain things, and it makes the transition to
QtWebEngine smoother.

doc/changelog.asciidoc

@@ -30,6 +30,7 @@ Breaking changes
 - New config system which ignores the old config file.
 - The depedency on PyOpenGL (when using QtWebEngine) got removed. Note
   that PyQt5.QtOpenGL is still a dependency.
+- PyQt5.QtOpenGL is now always required, even with QtWebKit.
 - Migrating QtWebEngine data written by versions before 2016-11-15 (before
   v0.9.0) is now not supported anymore.
 - Upgrading qutebrowser with a version older than v0.4.0 still running now won't

qutebrowser/misc/earlyinit.py

@@ -263,6 +263,7 @@ def check_libraries():
                          pip="PyYAML"),
         'PyQt5.QtQml': _missing_str("PyQt5.QtQml"),
         'PyQt5.QtSql': _missing_str("PyQt5.QtSql"),
+        'PyQt5.QtOpenGL': _missing_str("PyQt5.QtOpenGL"),
     }
     _check_modules(modules)
 
@@ -278,7 +279,6 @@ def check_backend_libraries(backend):
         modules = {
             'PyQt5.QtWebEngineWidgets':
                 _missing_str("QtWebEngine", webengine=True),
-            'PyQt5.QtOpenGL': _missing_str("PyQt5.QtOpenGL"),
         }
     else:
         assert backend == usertypes.Backend.QtWebKit, backend