When installing a graphics pack, confirm before installing DFHack init files

[Pidgeot]

I've recently received reports that some graphics packs include DFHack onLoad.init scripts which have been causing various problems by adjusting TwbT settings.

We should probably have users confirm before installing new DFHack init files in graphics packs, and possibly also for mods (though it's more normal there).

[Issue created by PeridexisErrant: 2017-06-18]

[jecowa]

The setting that's the biggest problem is twbt redraw_all 1. It forces everything to be redrawn every frame, which fixes issues with tiles shifting around, but it also makes the game unstable. Maybe give a special warning if this is present, and maybe the option to disable just this one line and allow the rest.

Settings like these are completely harmless (just changes the color and opacity of multi-level rendering overlays for when it's enabled):

multilevel shadowcolor 0 0 0 0.6
multilevel fogcolor 0.1 0.1 0.17
multilevel fogdensity 0.30 0.25 0.75

This is the most common use of the onLoad*.init file: twbt unit_transparency 1. I think this is safe and allows creature graphics to have transparent backgrounds. And the twbt workshop_transparency 1 is probably safe too– it makes the floors of workshops invisible.