and return a value.

in all .c files that needed it.

cucul_import_canvas().
  * Renamed cucul_create_export() into cucul_export_canvas() for consistency.

* Removed most code from the raw driver and replaced it with a call to the
    caca exporter.
  * Slightly changed the caca export format and updated code accordingly.
  * Improved cacaserver error reporting.

* Renamed canvas handle variables from "c" to "cv". Eh ouais mon gros.

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.

remove redundencies in the Doxygen documentation.

single frame that was saved by the raw driver.