mat: add non-zero doers for TriBandDense

[kortschak]

Please take a look.

[codecov-commenter]

Codecov Report

Merging #1810 (520a77a) into master (a2c6f81) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master    #1810   +/-   ##
=======================================
  Coverage   73.42%   73.43%           
=======================================
  Files         524      524           
  Lines       61754    61782   +28     
=======================================
+ Hits        45343    45369   +26     
- Misses      13854    13855    +1     
- Partials     2557     2558    +1     

Impacted Files	Coverage Δ
mat/triband.go	51.52% <85.71%> (+4.76%)	⬆️
graph/coloring/coloring.go	90.33% <0.00%> (+0.60%)	⬆️

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