Allegro 4's triangle() implementation is broken on OS X platform

[raynebc]

Elias and I communicated in the Allegro IRC channel:
https://pastebin.com/raw/rywdE7wL

We reached the conclusion that the logic no longer works in OS X at least as of High Sierra, specifically the use of _mac_sys_triangle() instead of the more generic polygon() function. I confirmed replacing my usage of triangle() with polygon() avoided rendering bugs when my program ran in High Sierra.

[guilt]

Thank you for using Allegro :)

Could you post a diff/PR? I doubt few people use it, but it would be good to have, if someone still wants to use the Allegro 4 library.

[raynebc]

I looked for a while but couldn't see where _mac_sys_triangle() was being inserted into whichever vtable made it the function called for triangle(). Somebody more familiar with the Allegro source code would be better able to do this.