[Intel-gfx] rfc: breaking old userspace gamma for 10-bit support

[Andrew Lutomirski]

AFAICT intel hardware wants a 129-entry LUT when using high precision
gamma ramps.  Rather than hacking some kind of decimation into the
kernel driver (and thus silently breaking DirectColor), I'd like to
teach userspace how to deal with variable gamma sizes.

gnome-color-manager already more-or-less supports arbitrary gamma ramp
sizes (supposedly), dispwin ought to do it, and there might not be any
other software that really cares.  gnome-screensaver saves and
restores the gamma ramp, and I haven't checked if it works right for
funny sizes.

The worst problem we'll have is that current xf86-drv-intel can't
handle non-256 gamma sizes at all.  So if we change the kernel we'll
break it completely.

One option is to have the kernel report gamma_size = 129 but still
accept 256 and decimate itself.  That might cause current userspace to
keep working (except for DirectColor).

Any thoughts?