ImGuiIntegration: use provided index offset

[pezcode]

Closes #86

[codecov]

Codecov Report

Merging #88 (b471f2c) into master (da3c1ab) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   77.25%   77.20%   -0.05%     
==========================================
  Files          21       21              
  Lines         941      939       -2     
==========================================
- Hits          727      725       -2     
  Misses        214      214              

Impacted Files	Coverage Δ
src/Magnum/ImGuiIntegration/Context.cpp	90.13% <100.00%> (-0.13%)	⬇️

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[jvannugteren]

Thanks. This fix solves the issues in my usecase.

[pezcode]

Same caveat as here: #90 (comment)

[mosra]

Thank you! Merged as 3e7b175.

For the record, we agreed on not supporting ImGui releases older than 2 years, so it's fine this change raises the minimum requirement to 1.71.