GrAtlas cleanup: Split out GrPlot and GrAtlas

GrAtlas cleanup: Split out GrPlot and GrAtlas

This breaks up GrAtlas into the head of the list (GrAtlas) and the list elements (GrPlot). It also moves all of the GrPlot management code into GrAtlasMgr. It adds a simple pool allocator for GrPlots and removes use of GrPlotMgr.

R=bsalomon@google.com, robertphillips@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/24981004

git-svn-id: http://skia.googlecode.com/svn/trunk@11508 2bbb7eff-a529-9590-31e7-b0007b416f81