[GUI] Antialising is compromised when items are hovered in the main menu

[pedropaulosuzuki]

Description

I suspect this is because we are using a filter over the final image, instead of tweaking the original colors. See how "Splitscreen Multiplayer" renders correctly, but "Singleplayer" doesn't.

Steps to reproduce

1. Launch SuperTuxKart
2. Hover your mouse over any of the main menu items

Configuration

STK 1.5 Flatpak

Error message

Log File (stdout.log)

No response

Input Configuration (input.xml)

No response