Qt5OpenGL: fix transparency issue with framebuffer objects with old OpenGL API

src/Gui/View3DInventor.cpp

@@ -588,6 +588,8 @@ void View3DInventor::previewFromFramebuffer(const QRect& rect, QImage& img)
     format.setAttachment(QtGLFramebufferObject::Depth);
 #if defined(HAVE_QT5_OPENGL)
     format.setInternalTextureFormat(GL_RGB32F_ARB);
+#else
+    format.setInternalTextureFormat(GL_RGB);
 #endif
     QtGLFramebufferObject fbo(rect.width(), rect.height(), format);
 #else