Support DDS Textures

[aussig]

The X-Plane format supports DDS textures. Note that X-Plane will try to load DDS textures if available irrespective whether the TEXTURE path points to a .png, so we should probably do the same.

[aussig]

Although this is implemented, we can't use it yet because of this bug: mrdoob#4316 where compressed DDS textures are displayed upside-down.

[aussig]

One solution is we switch off texture flipping completely (it is on by default), which would apply to all textures including PNGs. We would then reverse our vertical texture coordinates in all cases.

[aussig]

Polygon textures need to be flipped too, because letters such as https://www.opensceneryx.com/polygons/airport/taxisign_parts/characters_yellow/p/ are appearing upside-down. Might be worth doing the same with lines as well, in case they ever include letters.

[aussig]

Polygons are now fixed. Lines didn't need any changes.