Add Cursor pseudo-encoding support (disabled for now).

To change the appearance of the cursor, we use the CSS cursor style
and set the url to a data URI scheme. The image data sent via the
cursor pseudo-encoding has to be encoded to a CUR format file before
being used in the data URI.

During Canvas initialization we try and set a simple cursor to see if
the browser has support. Opera is missing support for data URI scheme
in cursor URLs.

Disabled for now until we have a better way of specifying settings
overall (too many settings for control bar now).

README.md

@@ -178,7 +178,9 @@ In the following table Jaunty is Ubuntu 9.04 and WinXP is Windows XP.
   is faster than Firefox 3.5, the high variability of web-socket-js
   performance results in overall performance being lower. Middle mouse
   clicks and keyboard events need some work to work properly under
-  Opera.
+  Opera. Also, Opera does not have support for setting the cursor
+  style url to a data URI scheme, so cursor pseudo-encoding is
+  disabled.
 
 
 ### Integration