Avoid some dict lookups and a few allocations

[rdeits]

Minor change on top of #100 which avoids a couple of extra Dict allocations by using get! instead of get

[codecov-io]

Codecov Report

Merging #101 into master will increase coverage by 0.38%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   73.45%   73.84%   +0.38%     
==========================================
  Files          12       12              
  Lines         324      367      +43     
==========================================
+ Hits          238      271      +33     
- Misses         86       96      +10

Impacted Files	Coverage Δ
src/visualizer.jl	75.34% <100%> (ø)	⬆️
src/lowering.jl	80% <0%> (-3.34%)	⬇️
src/geometry.jl	94.28% <0%> (-1.37%)	⬇️
src/MeshCat.jl	40% <0%> (+9.56%)	⬆️

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