RandR's QueryOutputProperty request makes the incredible decision of
throwing a BadName if you pass a property that doesn't exist, which
means that trying to check if a property exists is a royal pain when
using Xlib.
XCB's interface is much more friendly about errors and not having global
state and things like that, so use that instead to query our backlight
property.
If the property doesn't exist, a BadName error will be generated. This
is a terrible API, but it's what we're stuck with. Use
RRGetOutputProperty instead.
The output_id is more of an opaque identifier for the monitor, based on
its underlying ID from the windowing system. Since we also use the term
"output_id" for the output's index, rename our use of the opaque cookie
"output_id" to "winsys_id".
