Enable QPlatformBackingStore::toImage() when QT_NO_OPENGL is defined

There are cases where you would want to grab the contents of a QWidget
with a QSurfaceType of Raster, and in the case of a -no-opengl build
there is no way to get the contents of a QBackingStore.  The toImage()
function was unnecessarily lumped into the QT_NO_OPENGL ifdef, so this
commit removes this truncation of the API.

Change-Id: I2644e3dc85339eecd8bae8b91f23ef2f29a7eee4
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/gui/painting/qplatformbackingstore.cpp

@@ -411,7 +411,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
 
     context->swapBuffers(window);
 }
-
+#endif
 /*!
   Implemented in subclasses to return the content of the backingstore as a QImage.
 
@@ -424,7 +424,7 @@ QImage QPlatformBackingStore::toImage() const
 {
     return QImage();
 }
-
+#ifndef QT_NO_OPENGL
 /*!
   May be reimplemented in subclasses to return the content of the
   backingstore as an OpenGL texture. \a dirtyRegion is the part of the

src/gui/painting/qplatformbackingstore.h

@@ -121,7 +121,9 @@ public:
     virtual void composeAndFlush(QWindow *window, const QRegion &region, const QPoint &offset,
                                  QPlatformTextureList *textures, QOpenGLContext *context,
                                  bool translucentBackground);
+#endif
     virtual QImage toImage() const;
+#ifndef QT_NO_OPENGL
     enum TextureFlag {
         TextureSwizzle = 0x01,
         TextureFlip = 0x02,