Temp canvas doesn't clear for rectangles or curves

[GoogleCodeExporter]

1. Select Rectangle tool
2. Draw with it
3. Try drawing with it a second time

The first time reveals a glorious blur of rectangle that disappears when the 
temp canvas is hidden again. Drawing a second rectangle reveals the trash 
left over from the first use of the tool.

Oddly, the same technique is used to preview ellipses and lines.

Original issue reported on code.google.com by psoplayer on 10 Dec 2009 at 5:53