Add display-autoscale option to miral::display_configuration_options()

[AlanGriffiths]

This adds an autoscaling option to miral's display configuration options.

So to scale to emulate, for example, 1080p use --display-autoscale=1080

The scaling is quantized to the nearest quarter pixel

[codecov]

Codecov Report

Merging #3065 (242f8ee) into main (1c20be3) will decrease coverage by 0.03%.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##             main    #3065      +/-   ##
==========================================
- Coverage   77.69%   77.67%   -0.03%     
==========================================
  Files        1056     1056              
  Lines       73449    73473      +24     
==========================================
  Hits        57069    57069              
- Misses      16380    16404      +24     

Files	Coverage Δ
src/miral/display_configuration_option.cpp	48.35% <12.50%> (-12.85%)	⬇️

... and 3 files with indirect coverage changes

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[Saviq]

Works :)