vdiffr 0.3.0

This release of vdiffr features a major overhaul of the internals to
make the package more robust.

Cross-platform reliability

vdiffr now works reliably across platforms:

• svglite is now embedded in vdiffr to protect against updates of the
  SVG generation engine.

• It also embeds harfbuzz to compute font extents and text boxes
  metrics. This makes SVG generation of text boxes consisent
  across platforms.

While this makes vdiffr much more robust, it also means you will have
to regenerate all your testcases with the new version of vdiffr. You
can expect very few future releases that will require updating
figures, hopefully once every few years.

Now that vdiffr has a stable engine, the next release will focus on
improving the Shiny UI.

Regression testing versus Unit testing

Another important change is that figure mismatches are no longer
reported as failures, except when the tests are run locally, on
Travis, Appveyor, or any environment where the Sys.getenv("CI") or
Sys.getenv("NOT_CRAN") variables are set. Because vdiffr is more of
a monitoring than a unit testing tool, it shouldn't cause R CMD check
failures on the CRAN machines.

Despite our efforts to make vdiffr robust and reliable across
platforms, checking the appearance of a figure is still inherently
fragile. It is similar to testing for errors by matching exact error
messages: these messages are susceptible to change at any
time. Similarly, the appearance of plots depends on a lot of upstream
code, such as the way margins and spacing are computed. vdiffr uses a
special ggplot2 theme that should change very rarely, but there are
just too many upstream factors that could cause breakages. For this
reason, figure mismatches are not necessarily representative of actual
failures.

Visual testing is not an alternative to writing unit tests for the
internal data transformations performed during the creation of your
figure. It is more of a monitoring tool that allows you to quickly
check how the appearance of your figures changes over time, and to
manually assess whether changes reflect actual problems in your
package.

If you need to override the default vdiffr behaviour on CRAN (not
recommended) or Travis (for example to run the tests in a particular
builds but not others), set the VDIFFR_RUN_TESTS environment
variable to "true" or "false".

Features

• vdiffr now advises user to run manage_cases() when a figure was
  not validated yet (#25).

• Fixed a bug in the Shiny app that prevented SVGs from being
  displayed in Firefox (@KZARCA, #29).

• manage_cases() gains an options argument that is passed to
  shiny::shinyApp() (@KZARCA).

• The Shiny app now has a quit button (@ilarischeinin).

• New VDIFFR_LOG_PATH environment variable. When set, vdiffr pushes
  diffs of failed SVG comparisons to that file.

• expect_doppelganger() now takes a writer argument. This makes it
  easy to use vdiffr with a different SVG engine. See ?write_svg for
  an example function. Packages implementing a different SVG engine
  should wrap around expect_doppelganger() to pass their custom
  writer.

• write_svg() is now an exported function. It provides a template
  (function arguments and return value) for SVG writer functions.

• manage_cases() no longer checks for orphaned cases when a filter
  is supplied. (Orphaned cases are figures dangling in the figs
  folder even though their original expect_doppelganger() has been
  removed from the tests.)

Life cycle

• The verbose argument of expect_doppelganger() is
  soft-deprecated. Please use the vdiffr failure log instead. It is
  created automatically when run under R CMD check in
  tests/vdiffr.Rout.fail, and should be displayed on Travis.

  You can also set the VDIFFR_LOG_PATH environment variable with
  Sys.setenv() to unconditionally (also interactively) log failures
  in the file pointed by the variable.

• add_dependency() is soft-deprecated without replacement.

• The user_fonts argument of expect_doppelganger() is defunct
  because it complicated the UI for no clear benefit. The fonts used
  to generate the SVGs are now hardcoded to Liberation and Symbola.