New issue by TOertel on void-packages repository

https://github.com/void-linux/void-packages/issues/12035
Description: ### System

* xuname:  
Void 4.19.36_1 x86_64 AuthenticAMD notuptodate rrFFFF
(Everything is up to date except the kernel)

* package:  
gnuplot-5.2.6_1 (at least)

### Expected behavior
Drawing lines with alpha color should show as translucent.

### Actual behavior
Drawing lines are always 100% opaque.

### Steps to reproduce the behavior
````
gnuplot

  plot x lw 10, -x lw 10 lc rgb "#77000000"
  plot x lw 10, -x lw 10 lc rgb "#FF000000"  
  plot x lw 10, -x lw 10 lc rgb "#00000000" 
````

These should all display differently, but display the same.  Doing `set term png; set output "test.png"` and re-running the above generate the same incorrect result.

Alpha channel for lines was added in 5.2, I haven't found any other examples of alpha not working on the internet, so it seems like it's just void (or perhaps just me).