Add the `ftgl_` to the C API.

[cirosantilli]

C++ has the namespace ftgl so it's fine, but C does not have a prefix like ftgl_, so functions will occupy the global namespace, and maybe collide with other libraries.

We could solve that with macros.

That would help with: https://github.com/rougier/freetype-gl/issues/115

[adrianbroher]

The API is currently broken and inconsistent in naming, exposing of data and steps required to create and render text/glyphs. Adding a prefix doesn't solve any of this and is probably wasted effort given the idea of merging freetype-gl, glyph and msdfgen in the future as mentioned in #111.