OpenGL: use CHECK_GL_ERROR() only when a context is bound.

Removes a large number of spurious GL errors left & right.

Change-Id: I0b5639478812982b39d3b88be2600dad5d3f1fd0

slideshow/source/engine/OGLTrans/generic/OGLTrans_TransitionerImpl.cxx

@@ -1230,11 +1230,12 @@ void SAL_CALL OGLTransitionerImpl::update( double nTime ) throw (uno::RuntimeExc
     if (isDisposed() || !mbValidOpenGLContext || mpTransition->getSettings().mnRequiredGLVersion > mnGLVersion)
         return;
 
-    CHECK_GL_ERROR();
     mpContext->makeCurrent();
+    CHECK_GL_ERROR();
 
     glEnable(GL_DEPTH_TEST);
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+    CHECK_GL_ERROR();
 
     if(mpTransition)
     {

vcl/opengl/gdiimpl.cxx

@@ -1863,9 +1863,9 @@ void OpenGLSalGraphicsImpl::endPaint()
         mpContext->AcquireDefaultFramebuffer();
         glFlush();
         mpContext->swapBuffers();
-    }
 
         CHECK_GL_ERROR();
+    }
 }
 
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