such as cucul_dither_t instead of struct cucul_dither.
* Made cucul_buffer_t an opaque structure and implemented the two getters
cucul_get_buffer_data() and cucul_get_buffer_size().
* Documented all missing functions and function parameters.
* Documented most font functions.
* Cleaned up cucul_render_canvas() and made it actually usable by external
programs. Removed ugly printf() debug calls.