Screensharing: support variable stride

Summary: We were ignoring stride higher than stride calculated as width * bpp, which I assumed will always be the case, but it turns out that some drivers might use bigger stride, which might be width * bpp + some additional padding.

Test Plan: All current clients still work and also we now support bigger stride we get from gbm_bo.

Reviewers: Plasma, cblack

Reviewed By: Plasma, cblack

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28875