This change allows the labeling results to be cached to an image following a map render. If the cached label result image can be reused for the next render then it will be, avoiding the need to redraw all layers participating in the labeling problem and resolving the labeling solution. Basically this means that canvas refreshes as a result of changes to any NON-LABELED layer are much faster. (Changing a layer which is part of the labeling solution still requires all labeled layers to be completely redrawn)
This feature was developed by Nyall Dawson (North Road)