[image_view2] Optimize image_view2 to decrease CPU load.
1) add ~skip_draw_rate to throttle redrawing.
2) use ros::spin if possible
Merge remote-tracking branch 'refs/remotes/origin/master' into garaemon-optimize-image-view2
Merge pull request #647 from garaemon/optimize-image-view2

[image_view2] Optimize image_view2 to decrease CPU load.