Add new skyline Rectanizer

Add new skyline Rectanizer

The skyline Rectanizer tracks the leading silhouette of the
currently packed rects and tries to fit the next rectangle
on top of that. This gives slightly better packing behavior
than the current log2 scheme.

Also shrank the size of the plots, which increases their
number from 18 to 32 and gives better cache behavior (fewer
purges and uploads).

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/25584002

git-svn-id: http://skia.googlecode.com/svn/trunk@11577 2bbb7eff-a529-9590-31e7-b0007b416f81