Add a commandline flag for selecting video render backend

[Min1123]

For certain scenarios the OpenGL renderer may be hopelessly slow or unusable, while the SDL renderer may be acceptable. Since this has to be changed while in the GUI, or by environmental flags, it can be difficult to switch.

Recommend a command line flag for setting OpenGL or SDL as the video renderer on program start.

[tildearrow]

I am surprised to hear that the OpenGL renderer runs worse than the SDL one.
What's your operating system and graphics card?

[Min1123]

I'm messing with it on a VisionFive2 SBC (RISC-V), which I currently have running a flavor of Arch Linux (https://archriscv.felixc.at/) kludged together with their dev build (July 2, 2023) (https://github.com/starfive-tech/VisionFive2/releases/), and I'm using it over X11 forwarding, which means it's using LLVMPIPE or SOFTPIPE via MESA3D on a chip that has no GPU support (yet), and is spending all 4 of its very prototype-y cores trying to do the OpenGL, and keeps dropping responsiveness to where it makes my remote Gnome think it crashed.

I'm also waiting on a better cooling solution to get here, I think it's thermal throttling as its overheating.

Furnace runs great on aarch64 on an M1 Mac Mini with Asahi Linux and the current OpenGL 3.1 though, for a slightly less exotic platform.