[sksg] Trim down sksg::Matrix size

Matrix nodes are most often not composed outside of the DAG.  Split into
a basic Matrix class and a (private) ComposedMatrix subclass.

TBR=
Bug: skia:8340
Change-Id: I3c182e10ae5fa20d0639c1031523dba3956b75bb
Reviewed-on: https://skia-review.googlesource.com/153883
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed