get rid of a few thousand unnecessary OpenGL calls

For each iteration of the (gtk) main loop we are dropping the yield
mutext which results in calling OpenGLContext::clearCurrent. That method
calls OpenGLContext::ReleaseFramebuffers which would call in the end
glBindFramebuffer for each framebuffer related to that context.

This would easily grow into the thousand OpenGL calls without doing any
work.

Change-Id: I209cc534fe58a5e11ef7df7f850a787916b8bd43
Reviewed-on: https://gerrit.libreoffice.org/16241Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-on: https://gerrit.libreoffice.org/18218Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>

vcl/source/opengl/OpenGLContext.cxx

@@ -1633,8 +1633,11 @@ void OpenGLContext::ReleaseFramebuffers()
     OpenGLFramebuffer* pFramebuffer = mpLastFramebuffer;
     while( pFramebuffer )
     {
-        BindFramebuffer( pFramebuffer );
-        pFramebuffer->DetachTexture();
+        if (!pFramebuffer->IsFree())
+        {
+            BindFramebuffer( pFramebuffer );
+            pFramebuffer->DetachTexture();
+        }
         pFramebuffer = pFramebuffer->mpPrevFramebuffer;
     }
 }