The raster effects engine in some places multiplied a negative number by the
unsigned Bpp value, and added it to a pointer. The intention was probably to
subtract, which doesn't happen because of the implicit conversion to unsigned.
However, with 32-bit pointers the value wrapped around, leading to the intended
result.

Convert the result to a signed integer so it will subtract when intended.

Index: dgen-1.23/ras-drawplane.h
===================================================================
--- dgen-1.23.orig/ras-drawplane.h	2007-12-09 20:27:28.000000000 -0200
+++ dgen-1.23/ras-drawplane.h	2007-12-09 20:27:41.000000000 -0200
@@ -67,7 +67,7 @@
   tile_line = tiles + xsize * yoff;
   scan = yscroll & 7;
 
-  where = dest + (xstart + (xscroll & 7)) * Bpp;
+  where = dest + (signed)((xstart + (xscroll & 7)) * Bpp);
 
   for(x = -1; x < w; ++x)
     {
Index: dgen-1.23/ras.cpp
===================================================================
--- dgen-1.23.orig/ras.cpp	2007-12-09 20:27:28.000000000 -0200
+++ dgen-1.23/ras.cpp	2007-12-09 20:27:41.000000000 -0200
@@ -546,7 +546,7 @@
       start = 24;
     }
   add = -2;
-  where = dest + (start * Bpp);
+  where = dest + (signed)(start * Bpp);
   for(x=-1; x<w; ++x)
     {
       if(!total_window)
@@ -611,7 +611,7 @@
 	      // x flipped?
 	      if(which & 0x800)
 		{
-		  where = dest + (xend * Bpp);
+		  where = dest + (signed)(xend * Bpp);
 		  for(tx = xend; tx >= x; tx -= 8)
 		    {
 		      if(tx > -8 && tx < 320)
@@ -620,7 +620,7 @@
 		      where -= Bpp_times8;
 		    }
 	        } else {
-		  where = dest + (x * Bpp);
+		  where = dest + (signed)(x * Bpp);
 		  for(tx = x; tx <= xend; tx += 8)
 		    {
 		      if(tx > -8 && tx < 320)