Display rendering stats such as FPS in the OGRE overlay.

What does this MR do?

Closes #115 (closed).

Adds a new adaptor to display FPS and triangle count in ogre windows.

⚠ The triangle count seems to be broken, I'm not sure if it's because we need to count triangle in all render targets or because it is truly broken in OGRE...

Fixes a crash when setting the volume visibility when no image has been loaded.

Removes the option to print stats in a console.

How to test it?

• Run OgreViewer
• You can optionally set renderMode=always in OgreGenericScene.xml's SRender service and test OgreViewer again.