x11-wm/xcompmgr: Add patch to fix shadow rendering

The patch has been laying in upstream bugzilla for quite some time, but is
used on some linux distribution packages, such as debian packages.

While here, add missing dependency on xext.

PR: 182002
Submitted by: Brandon Gooch
Obtained from: https://bugs.freedesktop.org/show_bug.cgi?id=46285 (patch)
MFH: 2018Q2