SUG: Add instructions to upgrade Mesa3D libraries

guide/ch_advanced_use.tex

@@ -452,6 +452,37 @@ \subsubsection{NMEA Device}
 hardware and other settings, e.g. real GPS sensor or network and WiFi
 detection.\footnote{\url{https://support.microsoft.com/en-us/windows/windows-location-service-and-privacy-3a8eee0a-5b0b-dc07-eede-2a5ca1c49088}}
 
+
+
+\subsection{Modernized MESA3D libraries (Software OpenGL on Windows)}
+\label{sec:ExtraData:Mesa}
+
+Stellarium uses hardware-accelerated OpenGL for rendering. On very old Windows systems without dedicated hardware, 
+or in special circumstances like virtual machines, hardware accelerated graphics may not be available, 
+and the \program{Mesa3D} software implementation of OpenGL is used. This can also be forced by the command-line option \command{-\/-mesa-mode}. 
+Rendering the graphics without hardware acceleration is of course much slower than accelerated graphics, but it's surely better than nothing.
+
+The built-in Mesa library provides support for OpenGL~3.0 only. This prevents users of Qt5-based versions from applying dithering. 
+On Qt6, also the \program{ShowMySky} skylight model (see section~\ref{sec:skylight:ShowMySky}) is then not available. 
+
+The \program{Mesa3D} project \footnote{\url{https://www.mesa3d.org/}} meanwhile (2023) provides at least OpenGL~4.5. 
+If you need it and have Administrator privileges on your Windows system, \newFeature{23.3}
+you can replace the built-in library using the following steps to install inofficial builds\footnote{\url{https://github.com/pal1000/mesa-dist-win/}}.
+
+\begin{itemize}
+\item Download the latest ``release-msvc'' package as you need from \url{https://github.com/pal1000/mesa-dist-win/releases} and unpack it somewhere.
+\item Copy \file{(x86|x64)/{libglapi.dll,libgallium\_wgl.dll,opengl32.dll}} to where the executable \file{stellarium.exe} is.
+\item Delete the existing \file{opengl32sw.dll}
+\item Rename the just-copied \file{opengl32.dll} to \file{opengl32sw.dll}.
+\end{itemize}
+
+\noindent Now running Stellarium in Mesa mode (with link from program menu or command line) should provide OpenGL~4.5.
+Note however that after this upgrade, the program runs with considerably ($\approx$35\%?) lower framerate. 
+You must decide whether this is really beneficial in your situation.
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
 \chapter{Command Line Options}
 %\label{command-line-options}
 \label{sec:CommandLineOptions}