Pens with an integrated eraser share the serial number with the
corresponding pen but differ in the tool ID. libwacom provides a
"paired_id" mapping to look up one from the other.
If the user brings the eraser into proximity the very first time the
tool is seen, the eraser's tool ID is stored in the keyfile instead of
the associated stylus id. This leads to otherwise wrong behavior since
erasers share the serial number with the stylus but don't have any
buttons, don't have any erasers themselves, etc.
Fix this by looking up a matching stylus when we see an eraser. This
effectively filters our CcWacomTool to always be the pen, which is an
inadvertent assumption anyway.
42b39cb956
