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tablet: handle a BTN_TOOL_PEN on top of BTN_TOOL_RUBBER

Peter Hutterer requested to merge whot/libinput:wip/wacom-tool-pen-confusion into main

The Wacom 524C device triggers a kernel bug in the InRange and Invert handling. Every time BTN_TOUCH is set/unset the device also sets/unsets BTN_TOOL_PEN even when we nominally have the eraser in proximity.

The event sequence effectively looks like this:

# on prox in
BTN_TOOL_RUBBER 1
-- SYN_REPORT ---
# on tip down
BTN_TOOL_PEN 1
BTN_TOUCH 1
-- SYN_REPORT ---
# on tip up
BTN_TOUCH 0
BTN_TOOL_PEN 0
-- SYN_REPORT ---
# on prox out
BTN_TOOL_RUBBER 1
-- SYN_REPORT ---

To work around this, bias our duplicate tool detection code towards the eraser - if we have an eraser in-prox already and the pen goes in-prox, ignore it and continue with the eraser. But if we have a pen in-prox and the eraser goes in-prox as well, force a prox-out for the pen and put the eraser in-prox.

Recording originally from https://github.com/linuxwacom/xf86-input-wacom/issues/186

cc @bentiss, @pinglinux, @jigpu

This sits on top of !723 (merged), only the top commit matters here

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