[x11] Name client pixmap instead of frame pixmap

Summary:
Since KDE 4.2 - 4.3 times, KWin doesn't paint window decorations on real
X11 windows, except when compositing is turned off. This leaves us with
a problem. The actual client contents is inside a larger texture with no
useful pixel data around it. This and decoration texture bleeding are
the main factors that contribute to 1px gap between the server-side
decoration and client contents with effects such as wobbly windows, and
zoom.

Another problem with naming frame pixmap instead of client pixmap is
that it doesn't quite go along with wayland. It only makes more difficult
to abstract window quad generation in the scene.

Since we don't actually need the frame window when compositing is on,
there is nothing that holds us from redirecting client windows instead
of frame windows. This will help us to fix the texture bleeding issue
and also help us with the ongoing redesign of the scene.

Test Plan: X11 clients are still composited.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25610