Add instructions for compiling with Direct3D 12 support on Windows

contributing/development/compiling/compiling_for_windows.rst

@@ -162,6 +162,83 @@ dependencies. Running it will bring up the Project Manager.
           :ref:`doc_data_paths_self_contained_mode` by creating a file called
           ``._sc_`` or ``_sc_`` in the ``bin/`` folder.
 
+Compiling with support for Direct3D 12
+--------------------------------------
+
+By default, builds of Godot do not contain support for the Direct3D 12 graphics
+API. 
+
+To compile Godot with Direct3D 12 support you need at least the following:
+
+- Visual Studio (follow the instructions above to install). Currently, we don't
+  support building with Direct3D 12 enabled when using MinGW. Support will be
+  added in the future if possible.
+- `The DirectX Shader Compiler <https://github.com/Microsoft/DirectXShaderCompiler/releases>`_.
+  The zip folder will be named "dxc\_" followed by the date of release. Download
+  it anywhere, unzip it and remember the path to the unzipped folder, you will
+  need it below.
+- `godot-nir-static library <https://github.com/godotengine/godot-nir-static/releases/>`_. 
+  We compile the Mesa libraries you will need into a static library. Download it
+  anywhere, unzip it and remember the path to the unzipped folder, you will
+  need it below.
+
+.. note:: You can optionally build the godot-nir-static libraries yourself with
+          the following steps:
+
+          1. Install the Python package `mako <https://www.makotemplates.org>`_
+             which is needed to generate some files.
+          2. Clone the `godot-nir-static <https://github.com/godotengine/godot-nir-static>`_
+             directory and navigate to it.
+          3. Run the following::
+
+              git submodule update --init
+              ./update_mesa.sh
+              scons
+
+Optionally, you can compile with the following for additional features:
+
+- `PIX <https://devblogs.microsoft.com/pix/download>`_ is a performance tuning
+  and debugging application for Direct3D12 applications. If you compile-in
+  support for it, you can get much more detailed information through PIX that
+  will help you optimize your game and troubleshoot graphics bugs. To use it,
+  download the WinPixEventRuntime package. You will be taken to a NuGet package
+  page where you can click "Download package" to get it. Once downloaded, change
+  the file extension to .zip and unzip the file to some path.
+- `Agility SDK <https://devblogs.microsoft.com/directx/directx12agility>`_ can
+  be used to provide access to the latest Direct3D 12 features without relying
+  on driver updates. To use it, download the latest Agility SDK package. You
+  will be taken to a NuGet package page where you can click "Download package"
+  to get it. Once downloaded, change the file extension to .zip and unzip the
+  file to some path.
+
+.. note:: If you use a preview version of the Agility SDK, remember to enable
+          developer mode in Windows; otherwise it won't be used.
+
+When building Godot, you will need to tell SCons to use Direct3D 12 and where to
+look for the additional libraries::
+
+    C:\godot> scons platform=windows d3d12=yes dxc_path=<...> mesa_libs=<...> 
+
+Or, with all options enabled::
+
+    C:\godot> scons platform=windows d3d12=yes dxc_path=<...> mesa_libs=<...> agility_sdk_path=<...> pix_path=<...>
+
+.. note:: The build process will copy ``dxil.dll`` from the ``bin/<arch>/``
+          directory in the DXC folder to the Godot binary directory and the
+          appropriate ``bin/<arch>`` file in the Godot binary directory.
+          Direct3D 12-enabled Godot packages for distribution to end users must
+          include the ``dxil.dll`` (and relevant folders if using multi-arch),
+          both for the editor and games. At runtime, the renderer will try to
+          load the DLL from the arch-specific folders, and will fall back to the
+          same directory as the Godot executable if the appropriate arch isn't
+          found.
+
+.. note:: For the Agility SDK's DLLs you have to explicitly choose the kind of
+          workflow. Single-arch is the default (DLLs copied to ``bin/``). If you
+          pass ``agility_sdk_multi_arch=yes`` to SCons, you'll opt-in for
+          multi-arch. DLLs will be copied to the appropriate ``bin/<arch>/``
+          subdirectories and at runtime the right one will be loaded.
+
 Development in Visual Studio
 ----------------------------