Remove 'magic constant' use of ~0U as invalid/skip/no result.

* In python it's not as quick to get ~0U since ints aren't unsigned or
  fixed size. Adding named constants makes it easier for people to use
  the right values, and C++ users can still pass ~0U.

renderdocui/Interop/StaticExports.cs

@@ -280,7 +280,7 @@ namespace renderdoc
 
                 nextIdent = RENDERDOC_EnumerateRemoteTargets(host_mem, nextIdent);
 
-                if (nextIdent == UInt32.MaxValue || prevIdent >= nextIdent)
+                if (nextIdent == 0 || prevIdent >= nextIdent)
                     break;
 
                 callback(nextIdent);