fix ply parser

[SimonDanisch]

This fixes a couple of issues I run into testing pbrt scenes with Makie.

[codecov]

Codecov Report

❌ Patch coverage is 95.04132% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.25%. Comparing base (bcaeb9a) to head (89475b6).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/io/ply.jl	95.04%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   75.46%   79.25%   +3.79%     
==========================================
  Files          11       11              
  Lines         758      829      +71     
==========================================
+ Hits          572      657      +85     
+ Misses        186      172      -14     

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