XF86VidModeSetGammaRamp documentation

[tsuraan]

The man page for XF86VidModeSetGammaRamp says to check the source for
info on how to use it.  Where can I find this source?  On my system,
the function is declared in X11/extensions/xf86vmode.h, which is a
member of the xf86vidmodeproto package, but that source tarball
doesn't contain any actual source files.  I can find references to
SetGammaRamp in the xorg-server source, but at that point the actual
arguments to the client-side function are lost, so I still have no
idea how to call the function.  Any pointers would be much
appreciated.