glbumper: Use png_sig_cmp() instead of png_check_sig()
glbumper: Use png_sig_cmp() instead of png_check_sig()

Fixes bug #613186

mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl