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.
The non-AA outline ellipse was drawn outside the bounding rectangle
unlike all other ellipses. This commit now scales it to match the
size of the other ellipse drawing modes (AA, filled, etc).
This commit adds draw_ellipse() and moves the shared code
for circles and ellipses to draw_ellipse_part().
draw_ellipse_part() can draw an entire circle in one call using
8-way symmetry and an ellipse in two calls using 4-way symmetry.
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.
