Implement multi-threaded picture playback via cloning.

The CL adds SkPicture.clone() which produces a thread-safe copy by
creating a shallow copy of the thread-safe data within the picture and
a deep copy of the data that is not (e.g. SkPaint).  This implementation
re-flattens the paints when cloning instead of retaining the flattened
paints from the recording process.

Changes were also needed to various classes to ensure thread safety

Review URL: https://codereview.appspot.com/6459105

git-svn-id: http://skia.googlecode.com/svn/trunk/src@5335 2bbb7eff-a529-9590-31e7-b0007b416f81

• core/SkColorTable.cpp[diff]
• core/SkPath.cpp[diff]
• core/SkPicture.cpp[diff]
• core/SkPicturePlayback.cpp[diff]
• core/SkPicturePlayback.h[diff]
• core/SkPictureRecord.cpp[diff]
• core/SkPictureRecord.h[diff]