Engine: Certain mirrors are rendered incorrectly in Polymost

Originally reported here: https://forum.zdoom.org/viewtopic.php?f=340&t=67621

If there are multiple mirror textures adjacent to the same "mirror sector" (separated only by vertices) then only one of the mirrors is rendered correctly. All others result in a glitchy HOM effect. This only happens in Polymost, both Classic and Polymer are unaffected.

Example: https://imgur.com/ciZF5X4 , https://imgur.com/82iEPTj

This problem was introduced in SVN r7736 (commit e18c561f).

Demo map: amass_mirrors.map