main-map needs its own stacking context

(closes #876)

The rules for stacking contexts are complicated
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context

What I think was happening here was:

Without an explicit z-index, the main map didn't actually establish a stacking context
for the z-index values to start making sense

When the user starts interacting, we set a transform3d on supersurface, which
forces a stacking context to appear

css/80_app.css

@@ -4118,6 +4118,7 @@ img.tile-debug {
     user-select: none;
     touch-action: none;
     -webkit-touch-callout: none;
+    z-index: 0;
 }
 .main-map * {
     touch-action: none;