checks for alpha value too

As bug https://bugs.kde.org/show_bug.cgi?id=397891 reports,
the tool did not warn about PNG image having color not matched in
kgm file although RGB triple was foundable because of alpha
channel not full (255).
This commits adds this checking.

Subscribers: aacid, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D17110