GraphicsMagick + [GFX][processor_effects] = '0' is set to '-1' just by entering the Install Tool
Whenever entering the Install Tool, the setting [GFX][processor_effects] is set to '-1'.
I started with the GraphicsMagick preset and altered the LocalConfiguration.php by hand, setting [GFX][processor_effects] to '0'. This works until I click again on the Install Tool module. Navigating within the Install Tool directly after changing the value, you have to re-enter the Install Tool completly.
I work-around this by setting the value in AdditionalConfiguration.php which takes precedence over LocalConfiguration.php, but ideally this should be fixed in the core.
Bug #81700 is probably related. I'm not sure if this happens with ImageMagick as well, because I only have access to GraphicsMagick in the setup where I encountered this.