devdraw: fix for OS X 10.8 (Mountain Lion)

In MacOS 10.8, the NSBitmapImageRep class appears to cache the specified
image data at the time of construction. As a result updates to the
backing memimage object do not get pushed to the screen in flushimg.

This patch creates the NSBitmapImageRep object over again for each
flushimg which would appear to fix the problem.

R=rsc
CC=plan9port.codebot
http://codereview.appspot.com/6443063
1 file changed