Fix incremental visibility filter handling.

When drawing incrementally, SkDebugCanvas can end up applying the
visibility overlay multiple times (resulting in a fade-to-white effect).

The CL also includes a minor/unrelated SkDebugCanvas cleanup.

R=robertphillips@google.com

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/103083004

git-svn-id: http://skia.googlecode.com/svn/trunk/src@12478 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed