input: reset pointer image on leave

[cmeissl]

when nothing is focused we need to reset the cursor image. this has been accidentally removed during touch re-factor.
before the touch re-factor the cursor would have been reset whenever the focus changes, but it should be fine to
only reset it when there is no focus at all.

From the spec:

When a seat's focus enters a surface, the pointer image is undefined and a client should respond to this event by setting an appropriate pointer image with the set_cursor request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 20.55%. Comparing base (a04d9ac) to head (5386601).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
+ Coverage   20.53%   20.55%   +0.01%     
==========================================
  Files         158      158              
  Lines       25867    25868       +1     
==========================================
+ Hits         5313     5317       +4     
+ Misses      20554    20551       -3     

Flag	Coverage Δ
wlcs-buffer	17.96% <0.00%> (-0.01%)	⬇️
wlcs-core	17.62% <0.00%> (-0.01%)	⬇️
wlcs-output	7.53% <0.00%> (-0.01%)	⬇️
wlcs-pointer-input	19.43% <100.00%> (+0.01%)	⬆️

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