tablet: log a bug when a tablet switches between tools directly

We expect the kernel to transition properly for us, e.g. BTN_TOOL_PEN goes to
0, BTN_TOOL_ERASER goes to 1. Two cases have surfaced recently where this
doesn't happen and debugging this takes time - so let's warn about it to make
it obvious.

Example 1: https://github.com/linuxwacom/libwacom/issues/70
Example 2: https://gitlab.freedesktop.org/libinput/libinput/issues/259



This is just a warning, nothing more. We should just handle that case
accordingly but that requires more effort.

Fixes #260

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>