OBS Studio 21.1.0

General Changes
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* Updated the Browser source to Chromium 63. This includes many important and critical updates to the Browser backend.

  - The new Chromium build for the browser source now supports APNG (animated PNG) image files.
  - Fixed a few bugs and a potential crash on shutdown with the browser source, with more fixes and improvements to come in coming updates.
  - Fixed a bug where the bit icons in certain donation alerts didn't load properly (fixed with the Chromium update for the browser source).

* Dockable windows can now be nested.
* Updated the program to use the latest Microsoft development toolchain (Visual Studio 2017) for the Windows version.

Bug Fixes
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* Fixed a bug where you could import a scene collection with an already-existing name.
* Fixed an issue where a spontaneous rare audio glitch could happen when encoding audio on Windows.  This was caused by a Microsoft audio encoder library which was used by default on Windows at bitrates between 96 and 192 kilobits per second; this library has now been replaced.
* Fixed a potential crash on startup.
* Fixed a few scripting-related crashes.
* Fixed a bug where setting a blank replay buffer prefix would reset to the default prefix when the program was restarted.
* Fixed a bug where the slideshow source would get stuck on the last image is the slideshow image list was cleared.
* Selection rectangles are no longer drawn for audio-only sources.
