Add support for flipped UV coord generation w/ uv-cube-map-* fns

[acron0]

https://github.com/thi-ng/ws-ldn-8/blob/master/day1/ex03/src/ex03/webgl04.cljs

If you use :flags :nesw (for example) on the aabb, the texture appears to map to the wrong faces. So, 'front' and 'back' are removed but the texture still labels two of the sides 'front' and 'back' rather than 'east' and 'west'. It appears to have something to do with which faces are omitted, because :flags :ewfb works fine.

[postspectacular]

Hi @acron0 - so I found the problem and it's not a bug in this project, but has to do with the texture setup in our workshop example. We simply need to flip the texture (by setting :flip true here) for correct operation. However, since the cam is located inside the cube, the UV coords need to be manually flipped horizontally (they will only be correct if viewed from outside the cube). I will rename this task to focus on adding an optional arg to the attr/uv-cube-map-* fns to take this use case into account...