[pgf-tikz] Release 3.1.6

[Henri Menke]

# Acknowledgements

This release stands in the name of the two contributors **Alexander
Grahn** and **Yukai Chou** without whom this release would not have been
nearly as great.  Thank you very much!

# BREAKING CHANGES

- In the last version,  in an attempt to fix updating `local bounding
  box` in a clipping scope the `\pgf at path@size at hook` in
  `\pgf at protocolsizes` was set to be executed unconditionally.
  Unfortunately, this broke all other uses of `local bounding box` and
  has been reverted.  If you need to use `local bounding box` in a
  clipping scope, use the `overlay` option.

- The recent Ghostscript version 9.53 has changed the primitives for
  transparency, blend mode, and transparency groups.  These are now
  supported by PGF and should in principle go unnoticed by the user.
  (Thanks Alexander Grahn!)

- `\pgfintersectionoflines` will now always return the intersection in
  the untransformed coordinate system.  This however requires an
  additional `\pgftransforminvert` which comes with a loss of precision
  and can potentially lead to `Dimension too large` errors in edge
  cases.

- PGF now supports the new hook management that will be introduced in
  LaTeX 2020/10/01.  While this should not lead to any noticeable
  changes, please look out for breakages with overlays and the `current
  page` nodes.  Please report problems on the PGF or LaTeX issue
  trackers.

# New features

You can read about these new features in the manual:

- PS-3 functional shading, opacity masks (fadings) and image masks for
  dvips. (Thanks Alexander Grahn!)

- The `dvisvgm4ht` driver developed by Michal Hoftich has been merged
  into PGF/TikZ.

- The `pgfparser` module has been slightly refactored such that it can
  be used without loading all of PGF.

- The order in which the inner styles are applied in a `\matrix` is now
  configurable. #867

- The file `pgfmanual-en-macros.tex` is developed specifically for
  typesetting the PGF manual, however, many other package developers
  have found it useful and made good use of it.  To this end, we now
  install `pgfmanual-en-macros.tex` into a directory that is searched by
  kpathsea such that developers no longer have to copy the file into
  their own distribution.

- The CI system was switched from Travis CI to GitHub Actions for better
  integration with GitHub and direct deployment of build artefacts to
  the status page.

# Removed features

- The `bbox` library introduced in PGF 3.1.5 was removed because the
  author decided to impose restrictions on redistribution which violates
  freedoms 2 and 3 of the [Free Software
  Definition](https://www.gnu.org/philosophy/free-sw.en.html), making it
  effectively non-free which is in contradiction with the GPLv2 +
  LPPLv1.3c terms that PGF/TikZ is distributed under.  I further
  recommend that if there are files containing `bbox` code left over
  from a previous version that these are removed to avoid potential
  legal issues.

# Bug fixes

Lots of bug fixes.  On GitHub you can click the commit hashes and the
issue numbers to get to the fix and the ticket, respectively.

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