optimize mirror()

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36738 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
fix for #4593

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36739 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
shaders re-use geometry buffers

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@36740 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e