Fixed incorrect resizing flags that would hide the button when the window was too small.
Fixed crash due to assertion error when loading models where some textures are missing.

The problem was that unload wasn't called in case of an error. I stand by the unload pattern, because you never know when an OpenGL context exists, but it has to be applied consistently. Right now, failure to load a drawer does not remove the item from the document. That probably needs to be fixed.
Remove newly added items for which the drawer could not be created.

This avoids "ghost" items that are present in the model but not shown anywhere.
Broken, disabled or failing unit tests are all fixed now.

Due to additional changes in the mesh verification, TRInDataStream had to be altered to always give an output if it's valid, even if that output is 0 bytes long. All this together fixes #50.