Prevent the selection canvas from drawing when it isn't visible.
Prevent the selection canvas from drawing when it isn't visible.

This patch improves performance on zooming by preventing the selection
canvas from drawing when it isn't visible.

R=jackhou@chromium.org

Review URL: https://codereview.appspot.com/73400043

jackhou-chromium/bitmapper,jackhou-chromium/bitmapper,jackhou-chromium/bitmapper