- Added << operators for FString that function exactly like +=, except they

are left-associative, so you can use them like you would with an ostream
  and append to a string in a single expression without any overhead from
  allocating temporary strings as would happen if you used the + operator.
  In other words, instead of this:
     string += "Some string " + "that is assembled" + " in parts";
  You can do this and be more efficient while still being just as readable:
     string << "Some string " << "that is assembled" << " in parts";
- Changed PCD_PRINTBIND to include the command in its output if it isn't
  bound.
- Fixed: ACS_ExecuteWithResultValue could not be used inside a script because
  DLevelScript::RunScript() was not reentrant, thanks to having a global stack.
  The stack should be local to each instance of RunScript.
- Fixed: rt_draw4cols() could get stuck in rare situations where it thinks it
  should be drawing something but doesn't. Since long-term I plan to just
  replace all the masked drawing the variants of maskwallscan, I'm not going
  to try and find the real cause and fix it there. Instead, it just detects
  the situation and bails out when it finds it.


SVN r492 (trunk)

src/r_drawt.cpp

@@ -861,6 +861,7 @@ void rt_draw4cols (int sx)
 		//			aBc
 		if (bad != 0 || maxtop > minbot)
 		{
+			int drawcount = 0;
 			for (x = 0; x < 4; ++x)
 			{
 				if (!(bad & 1))
@@ -869,15 +870,23 @@ void rt_draw4cols (int sx)
 					{
 						hcolfunc_post1 (x, sx+x, horizspan[x][0], horizspan[x][1]);
 						horizspan[x] += 2;
+						drawcount++;
 					}
 					else if (minnexttop > horizspan[x][0])
 					{
 						hcolfunc_post1 (x, sx+x, horizspan[x][0], minnexttop-1);
 						horizspan[x][0] = minnexttop;
+						drawcount++;
 					}
 				}
 				bad >>= 1;
 			}
+			// Drawcount *should* always be non-zero. The reality is that some situations
+			// can make this not true. Unfortunately, I'm not sure what those situations are.
+			if (drawcount == 0)
+			{
+				return;
+			}
 			continue;
 		}