Blender Git Loki

Cleanup: Split image engine into ImageEngine, SpaceAccessor and DrawingMode.

Image engine is used to draw an image into a space. The current
structure wasn't clear and couldn't be easilly extended. This refactor
spliced the image draw engine into 3 main components.

- Space accessors: contains an interface to communicate with space data
(Image editor, UV Editor, Node Editor) in a common way. This reduced
the branching in the code base.
- DrawingMode: contains an interface to the used tactic to draw an image
inside the space framebuffer. Currently only one mode is implemented;
in the future there could be a separate drawing mode for huge images.
- ImageEngine: the core that connects the draw manager with the space
data and drawing mode.