This effectively creates a double-buffer for tools to use when modifying
the layer's bitmap (content or mask). Once the changes have been made
the tool reports to the layer that it has made changes along with a Rect
of the changed region. The layer will then merge the changes from the
scratch image to the real bitmap. This merge is done as follows: If a
given pixel is inside the selected region, the pixel from the scratch
bitmap is copied to the real bitmap. If the pixel is not inside the
selected region, the pixel from the real bitmap is copied to the scratch
bitmap.
As an optimization, when there is no selection active, the new method
for getting the scratch bitmap will return the real bitmap and no
merging will need to take place.
The Undo/Redo actions now tell you what kind of action will be
undone/redone. This is achieved by adding an "action text" field to the
ImageUndoCommand and having everyone who calls did_complete_action()
provide this text.
This is in preparation to support masking of Layers. We now distinguish
between the "display_bitmap" which will be the whole Layer with every
effect applied and the "content_bitmap" which contains the actual
unmodified pixels in the Layer.
