Integrate bgfx callbacks

[ikeycode]

Major ticket items here:

• Shader cache

Despite needing AoT 'compiled' shaders, they're still going to be dynamically recompiled on the target platform. Use bgfx callbacks to implement shader caching if enabled in the context. We should ideally expose a set of directories here, i.e. application name for XDG config/cache directories.

• Builtin screen capture support

Another nice feature of bgfx callbacks - we can screen-capture at will. Don't hotkey it, just expose the API. Ideally support automatic filenames here.