Make gm tool able to read and write image files in the same run

BUG=http://code.google.com/p/skia/issues/detail?id=571
Review URL: https://codereview.appspot.com/6052050

git-svn-id: http://skia.googlecode.com/svn/trunk@3717 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed