"Polyglot" might work better. I realize that these are not different languages that the library can use, but you could reasonably interpret it to mean that the library "speaks" SVG / Canvas / Javascript objects.
It's a tough one, because technically I think polyvalent may actually be the most correct word by definition (albeit from a different domain), but this thread would indicate that understandability and not technical correctness is more important.
Polyglot, as has been suggested elsewhere, probably fits best even if it's very slightly inaccurate (normally pertaining specifically to multi-language use)