Formspec marker API fixes

- Fix marker count not properly incrementing
- Fix registration of ids with : in their names

map_api.lua

@@ -134,14 +134,31 @@ function cartographer.register_biome(name, textures, min_height, max_height)
     };
 end
 
+-- Format marker ids to allow their use as formspec element ids.
+-- We're mostly concerned with guarding against the : character because it is
+-- common for ids and has an alternate meaning in formspecs.
+-- id: The id to format
+--
+-- Returns the formatted id
+local function format_marker_id(id)
+    return id:gsub(":", "_");
+end
+
 -- Register a marker with textures to display
 -- id: A string containing the id of the marker
 -- name: A string containing the displayedname of the marker
 -- textures: A table of texture names.
 --           These should correspond with detail levels,
 --           any detail level past the length of the table will return the last texture
+--
+-- Returns the final id that the marker was registered under
 function cartographer.register_marker(id, name, textures)
-    if not _cartographer.marker_lookup then
+    if not id then
+        return nil;
+    end
+
+    id = format_marker_id(id);
+    if not _cartographer.marker_lookup[id] then
         _cartographer.marker_count = _cartographer.marker_count + 1;
     end
 
@@ -149,6 +166,8 @@ function cartographer.register_marker(id, name, textures)
         name = name,
         textures = textures,
     };
+
+    return id;
 end
 
 function cartographer.is_filled(map, x, z)
@@ -180,6 +199,11 @@ end
 -- detail: The detail level
 -- Returns a string with a texture name, or nil if no matching marker was found.
 function cartographer.get_marker_texture(id, detail)
+    if not id then
+        return nil;
+    end
+
+    id = format_marker_id(id);
     local marker = _cartographer.marker_lookup[id];
 
     if marker then