Segfault when quickly adding image displays

[Martin-Idel-SI]

Adding a camera display and quickly adding a few image displays (use "duplicate") can result in an uncaught exception on both Windows and Linux.

This is probably a resource issue with Ogre: The call stack indicates that a new HardwareVertexBuffer cannot be created. Ogre only supports a number of render windows and OpenGLContexts. There is probably not much we can do about this, I just wanted to document the issue here.