This is a modified version of a patch we've been carry-ing in Fedora and
RHEL for years now. This patch automatically adds secondary GPUs to the
master as output sink / offload source making e.g. the use of
slave-outputs just work, with requiring the user to manually run
"xrandr --setprovideroutputsource" before he can hookup an external
monitor to his hybrid graphics laptop.

There is one problem with this patch, which is why it was not upstreamed
before. What to do when a secondary GPU gets detected really is a policy
decission (e.g. one may want to autobind PCI GPUs but not USB ones) and
as such should be under control of the Desktop Environment.

Unconditionally adding autobinding support to the xserver will result
in races between the DE dealing with the hotplug of a secondary GPU
and the server itself dealing with it.

However we've waited for years for any Desktop Environments to actually
start doing some sort of autoconfiguration of secondary GPUs and there
is sti...