move ctx.font string to the state struct

[chearon]

I don't totally understand why we were using a Nan::Persistent<v8::Value> instead of storing a standard string + turn it into a v8 value on the fly, so hopefully this is right.

fixes #1946

[chearon]

Ah, @zbjornson you fixed it the same way for other properties in 0e6504a

[zbjornson]

My only guess is that someone wanted to avoid the overhead of going between a V8 string and a std::string, but this branch improves performance of get font by about 10% and has no impact on set font.