I have never done it before( no script know-how): how I make it executable?
You can make any file executable via the filemanagers properties dialog.
So right click the file go to properties. In the newly opened window click the second tab called Permissions
There you should see a checkbox which you can check to mark this file as executable.
As this is a general issue with Qt nouveau and OpenGL I would suggest to activate this globally.
Other applications that use QtWebEngine like Konqueror, Akregator, Falkon are definitely also affected.
However if you don’t want to activate it globally you could create a new “link to application” on the desktop via using the right click new menu entry and entering the whole command in the program tab executable line:
LIBGL_ALWAYS_SOFTWARE=1 QT_XCB_FORCE_SOFTWARE_OPENGL=1 kmail
Don’t forget to set the Name like KMail and icon on the first tab of this dialog.
If you confirmed that this desktop link is working you can move the file to your home directories .local/share/application directory. The .local directory is a hidden one like every file starting with a dot at the beginning. To make it visible use the toolbars show hidden files button or press ALT+.
Hope that is all understandable