set the viewport before flushing

src/common/detail/clglcontext.cpp

@@ -83,9 +83,9 @@ void CLGLContext::end() {
     GL_CHECK(glBindFramebuffer(GL_FRAMEBUFFER, 0));
     //glFlush seems enough but i wanna make sure that there won't be race conditions.
     //At least on TigerLake/Iris it doesn't make a difference in performance.
+    GL_CHECK(glViewport(viewport_[0], viewport_[1], viewport_[2], viewport_[3]));
     GL_CHECK(glFlush());
     GL_CHECK(glFinish());
-    GL_CHECK(glViewport(viewport_[0], viewport_[1], viewport_[2], viewport_[3]));
 }
 
 void CLGLContext::acquireFromGL(cv::UMat &m) {