Detect decals after scaling happens, use glPolygonOffset instead of m…

[assertivist]

…essing with the depth buffer by hand

To my eyes, it fixes #26, at least I don't see it in all the usual places. This, unlike other things I've tried, does the job without distorting the rendering of ramps and other 'thin' objects that don't really need adjustment. That could also be due to the improvement on how the 'thin' objects are detected, I wasn't waiting until after the geometry had been put into its final scale. glPolygonOffset worked suspiciously well to fix this with whatever values I tried in it

[rherriman]

There are some lingering issues with this, at least on my machine. @assertivist has been unable to reproduce this specific issue, but occasionally faux shadows are still disappearing, seemingly when the camera is nearing a perpendicular viewing angle to them.

Occasionally decals are still not visible in the overhead view before the game begins. I can reproduce this reliably on Eulogy. Interestingly, adjusting the game resolution to a 4:3 aspect ratio makes them appear again.

That said, this is still a very significant improvement on the linked issue. Works for me.