Speed up invert colors effect

Summary:
Signed-off-by: André Agenor <andreagenor@icloud.com>
If all channels are selected a native Qt function is called, it is faster than calculating all channels and pixels in a loop
This is 97% faster than before

Reviewers: tcanabrava

Reviewed By: tcanabrava

Subscribers: pino

Differential Revision: https://phabricator.kde.org/D20334