SwiftWin32: extract the ScaleClient(rect:for:_)

[compnerd]

This refactors the DPI scaling for the rectangle to a function that we can use more broadly internally. The broader availability prepares us for fixing higher DPI displays.

[codecov]

Codecov Report

Merging #761 (75e52cc) into main (387cea8) will decrease coverage by 0.17%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##             main     #761      +/-   ##
==========================================
- Coverage   27.18%   27.01%   -0.17%     
==========================================
  Files         130      130              
  Lines        5385     5385              
==========================================
- Hits         1464     1455       -9     
- Misses       3921     3930       +9     

Files Changed	Coverage Δ
Sources/SwiftWin32/Views and Controls/View.swift	40.83% <42.85%> (-0.62%)	⬇️
Sources/SwiftWin32/CG/Rect.swift	85.71% <85.71%> (ø)

... and 1 file with indirect coverage changes