[svg] Add support for preserveAspectRatio

This fixes the aspect ratio for pretty much all tests.

Since we're going to rebaseline everything, also have dm use a white
background (to match other user agents).

Bug: skia:10842
Change-Id: Iab2afd61560af540539c216d1c3673f19fe0fe51
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328982
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
10 files changed