change SkSurface::newCanvas() to getCanvas(), and redefine its life-cycle to be

change SkSurface::newCanvas() to getCanvas(), and redefine its life-cycle to be
a single canvas for the lifetime of the surface.

Get a playback copy from the src picture, so we can continue to record into the
original picture.

git-svn-id: http://skia.googlecode.com/svn/trunk@4842 2bbb7eff-a529-9590-31e7-b0007b416f81