mrv2 v0.3.1

[ggarra13]

v0.3.1

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• Linux Binary releases that work on Rocky Linux 8, RedHat 8 and Ubuntu 18.04.

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• Added a Dockerfile for easy building and disting on all Linux distros.
  The base distro it builds on is Rocky Linux 8.

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• Updated manual building documentation for Rocky Linux, Ubuntu, macOS and
  Windows separately to make it clearer.

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• The main executable is mrv2.exe (Windows) or mrv2.sh (Linux / macOS ).
• Added getting all .so dependencies in CMake to distribute the executable
  appropiately.
• Changed hard-coded file extensions to use Darby's IO plugin system.
• Fixed crash on Linux GNOME when using native file requester.
• Added tooltips to Read Ahead/Read Behind caches to clarify they are in
  seconds.
• Added single click playback and stop on the view window, like RV.
• Added Doxygen documentation (very incomplete).
• Added displaying of spherical environment maps in a virtual sphere
  ( courtesy of an open source OpenRV shader for The Mill ).
• Added pen size change thru hotkeys.
• Fixed thumbnail creation on Windows.
• Removed memory leak of thumbnail creation.

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This discussion was created from the release mrv2 v0.3.1.