Bug fix #34713

Using MultiSample, the code that inspects Z buffer values fail, like for grease pencil
drawing on "Surface".

It appears to me that all this depth draw/read code is ignoring gpu offscreen entirely.
This should be confirmed by Campbell though :)

I'm also quite unsure why this code exists, related to other tools that use
depth too (particles, sculpt). Good to mark down for our viewport recode.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55462 954f8c5b-7b00-dc11-b283-0030488c597c