Opening the material selector after reloading files could cause long UI
freezes, because some linked in materials don't have the preview stored
in the source file. So Blender would keep rerendering it after every
file load, which may involve compiling OpenGL shaders, which again
freezes the UI typically. This was reported as quite an issue for the
Heist Production by the Blender Studio.
Don't render these missing material previews from linked data-blocks
anymore.
Differential Revision: https://developer.blender.org/D16538
Reviewed by: Brecht Van Lommel, Jeroen Bakker
571f373155