backend: fix resize factor calculation

[absolutelynothelix]

the corresponding regions need to be resized once for each window in the stack above the damaged window including the damaged window itself. we were off by one.

fixes #1088

[codecov]

Codecov Report

Merging #1095 (e8477e0) into next (4a39423) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             next    #1095      +/-   ##
==========================================
- Coverage   37.56%   37.38%   -0.18%     
==========================================
  Files          49       49              
  Lines       11190    11190              
==========================================
- Hits         4203     4183      -20     
- Misses       6987     7007      +20     

Impacted Files	Coverage Δ
src/backend/backend.c	60.91% <100.00%> (ø)

... and 3 files with indirect coverage changes

[yshui]

thanks. i'll make a note that we need to look at damage resizing again in the future. the region that needs to be repaint for blur is different for each level in the stack, but right now we are using the same one for all windows.