Blender Git Loki

UI: Only show render result in image editors in view mode

Previously, with the render display mode set to "Image Editor", we'd use
any image editor that doesn't already show a (non-render-result) image,
even if they weren't set to view mode (but UV, paint or mask mode).
It could be confusing or annoying when using an Image Editor for a
purpose that the mode wasn't created for.
Note that with the introduction of a UV sub-Editor, the old behavor was
even more confusing. Changing a UV Editor to show the render result was
weird.