This setting is only used when an application wants to control which samples are written to.  If it is disabled, multisampling should just write to all samples.

BUG=38

Review URL: http://codereview.appspot.com/2194043

git-svn-id: https://angleproject.googlecode.com/svn/trunk@431 736b8ea6-26fd-11df-bfd4-992fa37f6226