[1.2.3005] Texture smoothing does not correctly account for transparency

[Hexicube]

Example of issue: https://i.imgur.com/Jgwvtzv.png (bionic icons mod)

Texture smoothing appears to be using a naive method where colour and alpha are separately combined, instead of using alpha to work out the colour. As a result, icons from that mod - which have a fully transparent white area instead of black (likely their art software's fault) - gain a white border due to this bug.

I've informed the author on how to fix it, but this is also clearly a vanilla code issue.