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A stylus device can either report MouseEvents or TouchEvents. Styli used
on built-in displays should report TouchEvents, whereas those used on
external digitizers should report MouseEvents. This way, moving the
stylus on an opaque digitizer will also result in mouse/hover movements.

The zcr_stylus interface did not previously have a way to report stylus
events that were MouseEvents. We add a zcr_pointer_stylus_v2 interface
that extends the wl_pointer protocol so that the following additional
details can be reported with MouseEvents:
- tool change (mouse, touch, pen, eraser)
- force change (pressure)
- tool tilt change

BUG=b:169094254
TEST=exo_unittests

Change-Id: I0c4ac8412e1d5f7f01911ce3e0acbe6f8fbcad4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426981
Auto-Submit: Prabir Pradhan <prabirmsp@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Prabir Pradhan <prabirmsp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810791}