Actual blockmap fix

MI's "fix" was a reversion of something that allowed 2.0 maps to use the entire blockmap. This MR reverts that fix and adds a proper fix to the issue of west/south edges of the blockmap not working as they should. Tested with a thokbarrier-less square map (all sides were solid) and with AGZ (objects are tangible all around the map, like they are in 2.1.14).

src/p_local.h

@@ -38,6 +38,9 @@
#define MAPBMASK      (MAPBLOCKSIZE-1)
#define MAPBTOFRAC    (MAPBLOCKSHIFT-FRACBITS)
+// Convenience macro to fix issue with collision along bottom/left edges of blockmap -Red
+#define BMBOUNDFIX(xl, xh, yl, yh) {if (xl > xh) xl = 0; if (yl > yh) yl = 0;}
+
// player radius used only in am_map.c
#define PLAYERRADIUS (16*FRACUNIT)