Add engine version requirements to renderRequirements (Issue#25)

[Richardpwe]

Adds an optional engine property to renderRequirements: a list of requirements for the rendering engine (e.g. CEF, Gecko) with a minimum required version (version.min). min is intentionally defined as a string so different version formats (CEF, Chromium, WebKit, etc.) are supported.

[dvdeurse]

This looks ok to me, but we probably need something similar in the Server API so that Renderers can expose their supported engines.

[Richardpwe]

That makes sense, I didn't thought about that.
In the current Server API we already have RenderCharacteristics on RendererInfo, which is meant to correspond to the renderRequirements field in the graphic manifest.

To mirror the new renderRequirements.engine[] in the graphics spec, I’d add an
engine?: { type: string; version: { min: string } }[] field to RenderCharacteristics,
so the structure stays 1:1 between manifest and Server API. That would allow control systems to match a graphic’s engine requirements against what a renderer actually supports.