vdiffr 1.0.0

This release includes two major changes:

1. The internal SVG engine has been updated. It is now lighter, more
   robust, with fewer dependencies. It also generates more correct
   SVG. The main user visible change is that points look smaller than
   with older snapshots. Because of this update you will have to
   regenerate all existing snapshots with the new engine.

2. The snapshot management system now uses testthat (see
   https://testthat.r-lib.org/articles/snapshotting.html). Most of the
   R and javascript code has been removed from vdiffr as a consequence.
   vdiffr now serves as a reproducible SVG generation engine for
   testthat snapshots.

Migration of existing snapshots

There are two steps to update your snapshots to vdiffr 1.0.

1. This step is optional. Install the github-only 0.4.0 version of
   vdiffr with remotes::install_github("r-lib/vdiffr@v0.4.0"). This
   release only contains the new SVG engine. Review the snapshot
   changes as usual with vdiffr::manage_cases().

2. Install vdiffr 1.0.0 from CRAN, delete the tests/figs directory,
   run devtools::test(), and then testthat::snapshot_review().

Other changes

• expect_doppelganger() now supports grid objects such as gtable
  and grob (#36).