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

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

Change-Id: I0b5639478812982b39d3b88be2600dad5d3f1fd0
Reviewed-on: https://gerrit.libreoffice.org/18422Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

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)
     {
@@ -1277,8 +1278,8 @@ void SAL_CALL OGLTransitionerImpl::viewChanged( const Reference< presentation::X
 
 void OGLTransitionerImpl::disposeTextures()
 {
-    CHECK_GL_ERROR();
     mpContext->makeCurrent();
+    CHECK_GL_ERROR();
 
 #if defined( GLX_EXT_texture_from_pixmap )
     const GLWindow& rGLWindow(mpContext->getOpenGLWindow());

vcl/opengl/gdiimpl.cxx

@@ -1874,9 +1874,9 @@ void OpenGLSalGraphicsImpl::endPaint()
         mpContext->AcquireDefaultFramebuffer();
         glFlush();
         mpContext->swapBuffers();
-    }
 
-    CHECK_GL_ERROR();
+        CHECK_GL_ERROR();
+    }
 }
 
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