Adds depth buffer access to post processing shaders (via use of the depth_tex uniform)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10025 387891d4-d844-0410-90c0-e4c51a9137d3
Ooops, small buffer collision. Need to load the depth buffer to texture unit 2.


git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10026 387891d4-d844-0410-90c0-e4c51a9137d3
...aaand cleanup


git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10027 387891d4-d844-0410-90c0-e4c51a9137d3