Don't make font::texture destructor inline

The destructor calls glDeleteTextures() and making GL calls from a header
would could break the build if system GL headers are included with
glbindify headers.
Groundwork for geometry shader pipeline + per text VBO's

Benchmarking showed that reusing one VBO for all text rendering is
not efficient even with GL_STREAM_DRAW flag. Using a geometry shader to
generate glyph verticies might also improve performance as there would be
less data per character to generate/upload/etc.