Add a render engine with an add-on

An add-on (Settings › Add-ons) can add an engine to the Engine list of Render a picture…. The full text for add-on authors is section 16 of the add-on specification; in short:

• The add-on registers an engine with an id that starts with its own, a name, optional settings and either a program to run or a function in JavaScript that draws the picture.
• An engine that runs a program needs the engine.run permission, shown before install and in Settings. It runs only in Studio; in the browser it shows as a file to save.
• The add-on names the program (a plain file name: never a shell), a script that is inside the signed add-on, and arguments made of placeholders and plain words. Studio finds the program or asks you to point at it once.
• The first time an add-on runs a program, Studio asks you in its own dialog: which add-on, which program, and that a script can do anything the program can. Only allow it for an add-on you trust. Your answer is remembered for that add-on and program.
• The package (the scene as USD and a settings file, with the glTF file when the add-on asks for it) and the add-on's script go into a fresh temporary folder, the program runs there, and the picture comes back marked with the add-on's engine name.