1s lines will now block a mobj's movement. Sliding not implemented yet.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2751 86ba831e-150c-0410-ab35-e06eaae15ed9
Fixed some weirdness where floor portals were able to change the ceilingz in the clipping context.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2753 86ba831e-150c-0410-ab35-e06eaae15ed9
Fixed a bug with MarkVector::clearMarks where the marks would be cleared from the 0 index instead of from the firstDirty index.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2754 86ba831e-150c-0410-ab35-e06eaae15ed9
Fixed a problem with 1s lines in floor/ceiling portals.

git-svn-id: http://mancubus.net/svn/hosted/eternity/branches/portal-branch@2755 86ba831e-150c-0410-ab35-e06eaae15ed9