Allow brushTool to have opacity.

Change canvas composition from 'copy' (default) to 'source-over' to take
opacity of brush into account when painting over previously painted sections on
the canvas.

BUG=
https://github.com/jackhou-chromium/bitmapper/issues/25
https://github.com/jackhou-chromium/bitmapper/issues/15

R=tapted@chromium.org

Review URL: https://codereview.appspot.com/193720043