Make GrLayerCache use multiple plots in its atlas

Until we have a recycling Rectanizer the atlas purging must occur at the GrPlot level. This CL breaks the atlas into four plots to give some room for purging (without trashing the entire atlas).

This is calved off of (Add atlased layer purging - https://codereview.chromium.org/367073002/)

R=jvanverth@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/397873004
2 files changed