loaders: PNM: Optimize Save()

This adds custom function to write ASCII byte into a file which is more
than five times faster than printf("%u", byte).

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: Filters: Add point filters.

Add point filters to API Coverage test and FiltersCompare test.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Ditherings: Set errno and fix abort.

* Set errno to EINVAL on unsupported dst pixel type

* Return 1 and set errno to EINVAL on unsupported src pixel type
  (instead of aborting in GP_Convert.gen.c on unsupproted conversion)

Signed-off-by: Cyril Hrubis <metan@ucw.cz>