far better for error checking.

without using environment variables..
  * Implement caca_get_display_driver() to return the current display's
    output driver.
  * Add a simple example program to test caca_create_display_with_driver().

cucul_attr_to_argb64() (previously _cucul_attr_to_rgb12bg,
    _cucul_attr_to_rgb12fg and _cucul_attr_to_argb4) in the official
    libcucul API.
  * Cleanup complete: libcaca no longer depends on "cucul_internals.h".

direct cv->chars and cv->attrs access by display drivers.

drivers no longer need to access the cv->refcount private member
    and __cucul_set_canvas_size() is no longer needed.

collisions due to symbol mangling.