[effects/screenshot] Upload data to xpixmap in chunks

Summary:
xcb_put_image doesn't handle big images well. Contrary to XPutImage, the
caller must manually split the data in such a way that each xcb_put_image
request doesn't exceed the maximum request length. Not doing so will result
in libxcb shutting down the connection.

CCBUG: 338489
CCBUG: 388182

Test Plan: Take a screenshot of an active fullscreen client on a 4K monitor.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21370