-Encapsulate the functionality of the openings array into a class MemoryPool, which allocates a large pool of memory and then delegates small blocks of memory of any data type when asked.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3901 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Replace drawseg_t::maskedtexturecol with a pointer to tallpost_t*. The first post in each masked seg column is saved to drawseg_t to hopefully make it easier to unify the column rendering functions for masked segs and for sprites.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3902 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Reject segs that are not facing the camera before bothering to clip the seg to the view frustum. Should be a small speed-up.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3903 b0bcef46-2027-0410-bd56-ab726e9a1d04
-Merge from trunk into rend32.

git-svn-id: http://odamex.net/svn/root/branches/rend32@3906 b0bcef46-2027-0410-bd56-ab726e9a1d04