Sandbags surfaces update

[jonpas]

When merged this pull request will:

• Add Tanoa surfaces support to Sandbags
• Check height above terrain, if more than 0.05 disallow sandbags (when on buildings and similar)
• Check if in water (ATL is negative during shallow swimmable water for some reason)
• Use private keyword

[PabstMirror]

Should we update the list in ace_trenches as well?
https://github.com/acemod/ACE3/blob/master/addons/trenches/functions/fnc_canDigTrench.sqf

[jonpas]

Oh, didn't even realize there is a function that's exactly the same. I'll move it to common even.

[PabstMirror]

Can build trench on stratis runway
surfaceType = "#GdtStratisConcrete"; CfgSurface -> concrete has dust of 0.15

Maybe we should keep the blacklist and just use the dust as a backup for external maps?
Or tweak dust cutoff?

[nateberkopec]

I have a crapton of mods/terrains loaded, noticing lots of concrete surfaces use 0.15 for dust. XCam-Taunus appears to use 0.15 for many of their road surfaces as well.

[jonpas]

Will look at it and possibly just keep dust as fall-back if higher value will cause other problems.

[jonpas]

Changed to fall-back for dust, as it does not work as expected for some cases.