Fix font API to use font ID instead of Box<T> about nuklear-rust HOT 2 CLOSED

See the solution as making the NkFontAtlas actually owning the NkFont pointers (stored in Vec<*mut NkFont>), as this actually happens at the C level. Otherwise I have to return a borrow with 'NkFont' which will have a lifetime while being stored, and this this will pollute all the user code.
NkFontID type, left of early versions, fits the purpose perfectly.

from nuklear-rust.

Ok, so far this will work by querying a font from atlas with FontAtlas::font(font_id). While this utilizes native library font storage and does not introduce segfaults, it has a performance drawback of traversing the linked list with fonts. So far the possible solution is to unsafely copy the font pointer to the user code and store it there, and don't forget to std::mem::forget() it before purge.

from nuklear-rust.