[Etoys] Screen damage issues when dropping tiles in ScriptEditor

[karl]

When I turn on preference showDebugDamage there are some
issues when dropping tiles in the ScriptEditor. A rectangle in the
top left corner flashes, and a large part of the screen is redrawn when
the tile is dropped, from top left of screen to bottom right of 
ScriptEditor.
I guess this will slow the tile scripting responsiveness.
Also the Viewer flap tabs are redrawn quite often, not flap tabs for 
SketchMorphs, but for other morphs.
Karl