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