The build was failing with an undefined reference to `_caca_alloc2d`.
This was because the function was internal to libcaca but used by
the `cacaview` utility.

This change makes `_caca_alloc2d` a public function `caca_alloc2d`
by moving its declaration to the public header `caca.h`. All call
sites have been updated to use the new function name.

This fixes the build and allows the Python bindings to be used.

split is rendered moot by the plugin system: when enabled, binaries do
not link directly with libX11 or libGL. I hope this is a step towards
more consisteny and clarity.

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.