Summary:
So far this connect was only in ShellClient, but it's needed for all
Toplevels. Also when a XWayland window has it's surface size changed,
the pixmap needs to be discarded. Otherwise KWin might hit an assert
in AbstractEGLTexture due to the buffer having a different size than
the texture had previously.

Test Plan:
Firefox, click download on bugreports.qt.io used to trigger
this for me. With this change no longer able to reproduce.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2583