Update layers.js to add support for FAA charts

[ThomDyson]

Adds support for FAA VFR Sectional, VFR Terminal, IFR Low, and IFR High charts from official FAA chart server. This restores most of the charts previously provided through chartbundle.

[wiedehopf]

So i've merged but disabled these layers.
As you can see i've made some changes to work around the tileservers not having all zoom levels available.

I don't suppose you can get in contact with whoever is in charge of these tile layers?
Requesting zoom 8 across the entire US causes a lot of requests.

You'll have to reenable with if (1) and see how the changes are doing.
Zoom lvls 1 to 7 are just returning 404.
For the high IFR map zoom level 6 is available but is a different map than higher levels.

[ThomDyson]

This service is the official US FAA tile service through ARCGis, so no chance of having them change their output. :) Since these are raster tiles, I can see the logic of limited value beyond specific zoom levels.
I don't know how to see the zoom level value that I'm viewing at, but I do see where the IFR High map adds in the trans-pacific route detail and removes the continental US detail. I can see the point of that transition too.

[wiedehopf]

Limiting at high zoom levels is perfectly fine.
Limiting the lower zoom levels as in the overview is very impractical.

Have you seen the minZoom and maxZoom settings i've put in the source code?
Normally minZoom is just 1 and you can zoom out and get an overview without loading a bunch of small tiles.
Without the minZoom there are 404s and you get a grey map when you zoom out.

The state it's in will cause a lot of load on the arcgis servers when zoomed out.

I guess ignore the complaints that it doesn't work properly and enable it anyhow.
This is gonna be annoying.

Enabled.