Use the most simple scene manager possible.

It seems as if the Octree scene manager when loaded will match all scene
manager requests, and we don't want to use that since it's too much
overhead for these simple scenes.
Update with new material names after shuffle.
Generate mesh tangents if missing.

The tangents can in almost all cases be generated from the existing mesh
data. But it takes some time, so it's preferred to have them
pregenerated.
By making this automatic we can easier work with development media. It's
expected that all release media should be processed, so tangents should
always be generated for that. If not, we'll log with a warning.
Ignore directories names "source".

The client shouldn't look into the "source" directories as we're only
interested in the final assets.
Fixed possible segfault.
Allow reload of mesh assets.