filters: ditherings: Cleanups.

Change the API of Floyd-Steinberg and Hilbert-Peano ditherings to match
rest of the functions function.

* Rename the functions
* Fix all usages
* Update list of filters exported symbols
* Fix python bindings
* Add ditherings to API Coverage tests

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Update dithering docs.

* Update filters C API pages

* Start python filter docs

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Update list of Text symbols.

Add GP_FontC64.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Cleanup check_symbols.sh, add Gamma syms.

Now check_symbols.sh prints ALL CAPS warning and instructions how to fix
the problem.

Also add the Gamma table symbols for now, so that the output from the
script is clean.

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