Treat a missing <diffuse> the same as a black diffuse color.
The easiest way to see this bug is with a Collada shader like
```
<constant>
<emission>
<color sid="emission">1 0 0 1</color>
</emission>
</constant>
```
The Collada spec says this should be just
```
color = <emission>
```
ie. red everywhere. The importer slots the red into the Principled Emission socket, but since it leaves the Base Color as the default off-white, this is added to red, and the material looks white-pink in the light and red only in the shadows.
Putting black in the Base Color makes it look red everywhere.
D10939 will also eliminate the much-less-noticeable specular term for this case.
Reviewed By: gaiaclary
Differential Revision: https://developer.blender.org/D10941
Collada shaders with black <specular> should import with Specular=0.
(A missing <specular> is the same as black.)
The general specular conversion is hard, but this case is common and easy.
Fixes the specular for all <constant>/<lambert> shaders, and <blinn>/<phong>
shaders with black/omitted <specular>. Before this they all looked too "shiny".
Reviewed By: gaiaclary
Differential Revision: https://developer.blender.org/D10939
An emission texture is currently connected to the Base Color socket. It should connect to the Emission socket, like a constant does.
Reviewed By: gaiaclary
Differential Revision: https://developer.blender.org/D10990
While rB6b18678e34bf did the correct updates, it did it a bit early (not
covering all possible node tree changes).
Now do the ntreeUpdateTree() at the very end of the material import
instead.
Thx @scurest investigating.
Maniphest Tasks: T86314
Differential Revision: https://developer.blender.org/D11159
Make sure we have valid links and link pointers using
'ntreeUpdateTree()'.
Maniphest Tasks: T86314
Differential Revision: https://developer.blender.org/D10793
This moves the `alembic`, `avi`, `collada`, and `usd` modules into a common
`io` directory.
This also cleans up some `#include "../../{somedir}/{somefile}.h"` by
adding `../../io/{somedir}` to `CMakeLists.txt` and then just using
`#include "{somefile}.h"`.
No functional changes.
