Alter layer hoisting to only hoist layers for one canvas at a time

This CL alters layer hoisting to defer creation of the free floating layers until they are actually needed (rather than creating _all_ the hoisted layers at the start).

It also fixes a pre vs. post Concat bug with how matrices were being accumulated.

BUG=skia:2315

Review URL: https://codereview.chromium.org/657383004
8 files changed