API changing improvements to initialization and text rendering, bug fixes

- moved init responsibility to the backend, glfonsCreate() now creates
the context
- text drawing and measuring now returns nextpos/width instead of
requiring pointer to width
- fixes to glyph padding so that it is possible to renderer rotated
text without garbage
- made rendering callbacks optional
- added "pull" interface to get one glyph at a time and to retrieve and
validate texture changes
- added name to add font