Add Brush::kMax/MinBrushSize constants
Optimize ToolLoop with modified regions of pixels

Here we avoid copying and clearing pixels that will not be used
in the whole tool loop process.
Changes:
* Add several member functions in ToolLoop to validate/invalidate regions
  of source/destination images so we know what regions are safe to use
  by inks and can be shown in the editor
* Add new DocumentObserver::onExposeSpritePixels() member to validate
  pixels that will be displayed in the editor
* Add Ink::needs/createSpecialSourceArea() member functions to validate
  extra areas for inks like blur or jumble
* Add undoers::ModifiedRegion to save the undo information about the
  modified region
* Add ShowHideDrawingCursor class
* Change "blur" tool policy from overlap to accumulate

(This is a real fix for issue #239)
Convert TODO item to issue #560
Critical fix in Image::clip_rects()