levels: map15 fixes
levels: map15 fixes

Bunch of fixes in map15:
* Player can be trapped at sectors 88 and 1014 if the player stays here
  too long
* Sector 181 has no items in easy skill levels
* Wrong lower front side texture on lines 460/437
* Player can get trapped in sector 30 and die
* Many door tracks don't lower unpegged
* Floor of sector 246 looks better with FLOOR5_3
* Wrong texture on line 1460
* Lines 712, 715, 1460, 1466, 1459, 5882, 5880, others need aligning
* Floor of sector 358 looks better with CEIL3_5
* Floor of sector 576 looks better with FLOOR0_3
* Line 7181 was upper unpegged; door texture did not scroll on opening

Signed-off-by: Mike Swanson <517655fde76fa2f07143b6c6f1774e4fae45930a@gmail.com>

CWolfRU/freedoom,CWolfRU/freedoom,jmtd/freedoom,jmtd/freedoom,jmtd/freedoom