vg: rename width_len.ext golden files

[sbinet]

Please take a look.

[sbinet]

this is a simple spinoff from #704 and #708 (intended to reduce the complexity of merging/rebasing branches)

[codecov-commenter]

Codecov Report

Merging #712 (97de8ef) into master (a58aa90) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #712      +/-   ##
==========================================
- Coverage   71.86%   71.84%   -0.03%     
==========================================
  Files          56       58       +2     
  Lines        4959     5171     +212     
==========================================
+ Hits         3564     3715     +151     
- Misses       1209     1261      +52     
- Partials      186      195       +9     

Impacted Files	Coverage Δ
vg/vggio/context.go	100.00% <0.00%> (ø)
vg/vggio/vggio.go	67.37% <0.00%> (ø)

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