windowthumbnail: Fix the GLXFBConfig selection code

Check that the sizes of the color channels in the fbconfig match those
in the window visual.

This fixes a 2-10-10-10 fbconfig being choosen for the ARGB32 visual.

Differential Revision: https://phabricator.kde.org/D10173
Signed-off-by: Fredrik Höglund <fredrik@kde.org>
Acked-by: David Edmundson <kde@davidedmundson.co.uk>