Fix cursorGuide alignment.
Fix cursorGuide alignment.

This fixes issue #91 where the cursorGuide is positioned 1px too far
down and right due to its border.

This also makes the brush tool's cursor no longer aligned to pixels
since when the brush draws, it does not pixel align its position.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/91
R=mgiuca@chromium.org

Review URL: https://codereview.appspot.com/229160043

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