[Bug - Proj Mesh3D]

[ftoromanoff]

Linked to the PR #2180 that changed the way base_alti is handled, we noticed a difference in the representation of object in the 3D environnement.
Take the examples source_file_gpx_raster and source_file_gpx_3d that both use a gpx file to draw it on a itowns scene. The first one project it on the raster layer when the second one draw it as a mesh. We exept the 2 way to show very similar view.
Or (already before) that's not the case moreover the newly merge PR make it even more different (see example following)

On master before merge:
tilt: 45

tilt: 0 (Nadir)

On master after merge:
tilt: 45

tilt: 0 (Nadir)

[ftoromanoff]

I propose a fix in PR #2223. Might not be the best in regard of efficiency but we get back the same result as we had in master.