Support fractional scale

[cmeissl]

This PR add support for wp_fractional_scale, rounding should be good for toplevel surfaces.
The rounding for subsurface position and size is explicitely undefined in the spec, so it should be
fine for now as is I guess.

Testing done with single-surface from updated test-clients

depends on wayland-rs#590 merged

[codecov-commenter]

Codecov Report

Base: 24.78% // Head: 24.57% // Decreases project coverage by -0.21% ⚠️

Coverage data is based on head (705c5f4) compared to base (91518e9).
Patch coverage: 10.93% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #840      +/-   ##
==========================================
- Coverage   24.78%   24.57%   -0.22%     
==========================================
  Files         124      125       +1     
  Lines       19969    20248     +279     
==========================================
+ Hits         4949     4975      +26     
- Misses      15020    15273     +253     

Flag	Coverage Δ
wlcs-core	22.52% <10.93%> (-0.20%)	⬇️
wlcs-output	9.70% <10.41%> (-0.04%)	⬇️
wlcs-pointer-input	24.35% <10.93%> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/desktop/wayland/utils.rs	45.24% <0.00%> (-0.15%)	⬇️
anvil/src/state.rs	37.91% <3.50%> (-6.42%)	⬇️
src/wayland/fractional_scale/mod.rs	13.74% <13.74%> (ø)
anvil/src/shell.rs	14.93% <100.00%> (ø)
src/lib.rs	24.50% <0.00%> (-75.50%)	⬇️
src/backend/renderer/damage.rs	67.18% <0.00%> (-3.36%)	⬇️
build.rs	78.94% <0.00%> (-1.06%)	⬇️
src/output.rs	68.91% <0.00%> (+2.02%)	⬆️

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