Revert "Revert "Fix Post Processing in 64-bit Software 3P/4P""

This reverts commit 995edfb1.

src/v_video.c

@@ -292,7 +292,7 @@ void VID_BlitLinearScreen(const UINT8 *srcptr, UINT8 *destptr, INT32 width, INT3
 #ifdef HAVE_VIDCOPY
     VID_BlitLinearScreen_ASM(srcptr,destptr,width,height,srcrowbytes,destrowbytes);
 #else
-	if (srcrowbytes == destrowbytes)
+	if ((srcrowbytes == destrowbytes) && (srcrowbytes == (size_t)width))
 		M_Memcpy(destptr, srcptr, srcrowbytes * height);
 	else
 	{