cg_api: make possible for game to know if an extension-independent image path exists

[illwieckz]

I wanted to know if it's possible for the game to query the engine to know if an extension-independent image path exists, it looks like this code make it possible. It would also be possible to do the same for sounds if we have a need.

What this make possible is for example to check if Unvanquished's meta/<name>/<mapname> levelshot exists, but if it does not exist, try to load Quake3-like levelshot/<name> instead.

[slipher]

Ignoring possible warning spam, is there some use case that you can't do by attempting to load the image and seeing if the handle is 0?