Replace CVT implementation with libxcvt

libxcvt is a library providing a standalone version of the X server implementation of the VESA CVT standard timing modelines generator.

libxcvt also provides a standalone version of the command line tool cvt copied from the Xorg implementation and is meant to be a direct replacement to the version provided by the Xorg server.

libcvt can currently be found here: https://gitlab.freedesktop.org/xorg/lib/libxcvt

#1142 (closed)