Texture bar: Minor fixes
Some issues with the texture bar:
- Option to have white background in the bar (so that mostly transparent textures are visible in front of other scene background)
- Crash when labels -> num ticks (or labels? can't remember) is set to 0
- texture aspect ratio is wrong if the data grid has a different aspect ratio (e.g. 2x1 degrees)
- Change name in NWPActorVariable from "spatial transfer function" to "textured transfer function"