Most of the 2D overlays in mitk are using their own vtkrenderer as a seperate layer. there are currently 4 rendering layers active which is not necessary because the concerned overlays are always on top anyway.

I implemented the following changes:

• Deprecate mitk::GradientBackground and set the Gradient colors directly in the scene renderer. There is no need for a background renderer.

• Improved vtkMitkRectangleProp:
  • better placement of lines, so that the border width can be set in exact number of pixels
  • no need for vtkRenderer as a member

• replaced mitk::RenderWindowFrame with direct usage of vtkMitkRectangleProp in the scene renderer (removed foreground layer)

• directly applied vtkCornerAnnotations to the scene renderer
  • removed foreground layer and improved performance because the cornerannotations were constantly created and added.

• restored the missing orientation arrows feature.