web: Add 'preferred renderer' config option

[n0samu]

Describe the feature

With the switch to wgpu-webgl, I think it's even more important that we add a config option to the web version of Ruffle that allows choosing the render backend:

• It makes it much easier to test the behavior of the different render backends. No need to manually disable WebGL 1/2 in the browser's config to force a fallback to the WebGL/Canvas backends.
• Some content does not run well with wgpu-webgl yet. For example, Bloons Super Monkey performs badly and uses excessive GPU resources when you buy the plasma tentacle upgrade because the blend mode that it uses is not well-optimized in Ruffle yet. Switching to the WebGL backend resolves the problem. (The blend mode barely affects how the tentacle displays)
• Some content is drawn more accurately with the Canvas backend due to a stroke width issue with other backends. See Excessive thickness (wide stroke) on shape outlines #7369 for an example.

The behavior I'd suggest for this is to keep the same fallback behavior as usual, but just 'start' at the preferred renderer. So if WebGL is chosen as preferred, it still falls back to Canvas if WebGL is not available.