Font fallback api in command_queue.hpp

As we've discussed offline, the command_queue.hpp is the api that we want to move towards.

However, I'm not sure how this CommandQueue::addGlobalFontAsset(std::string name, FontHandle handle) should work for fonts.
In my use case, I have two use cases.

• out-of-band/referenced fonts
• font fallbacks

If it's already possible, can you provide examples of how to do this?

1. After loading a .riv, the user wants to swap one font in the FileHandle.
2. After loading a .riv file, and setting a localised text, the text uses glyphs that aren't present in the default font. Adding one or more font fallbacks is needed.

Suggestions:

• Add CommandQueue::addFont(FileHandle file, std::string name, FontHandle handle, uint64_t requestId = 0) - Replaces the font asset with name with a new font.

• Add CommandQueue::removeFont(FileHandle file, std::string name, FontHandle handle, uint64_t requestId = 0) - Removed the font asset with name with a new font. Leaves it empty, as one has to be able to explicitly control memory usage.

• Add CommandQueue::addFontFallback(FontHandle font, FontHandle fallback, uint64_t requestId = 0) -- adds a font fallback (e.g. japanese) to an existing font

• Add CommandQueue::removeFontFallback(FontHandle font, FontHandle fallback, uint64_t requestId = 0) -- removes a font fallback from an existing font

Start with command_queue.hpp and inspect the existing CommandQueue, FileHandle, and FontHandle APIs. Compare the requested font replacement, removal, and fallback operations with the two stated use cases; done means the API behavior and usage examples are agreed and implemented.