don't add vml namespace if already done

[pgiraud]

We shouldn't add the vml namespace to the document if it has already done or if namespace already exists. For example, if we add several vector layer to the map, each will add the "v" namespace and will create a new style sheet rule.

Though, I don't really know what happens if user already set a vml namespace in the HTML document but with no proper or wrong style sheet rules.

Attached patch proposes to test if namespace already exists before adding it with corresponding style sheet rule.

[crschmidt]

(In [5236]) Only add the VML namespace if we haven't already added it. (Closes #1141)